diff --git a/DESCRIPTION b/DESCRIPTION
index 59c1b93..c2c0a96 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -13,7 +13,7 @@ Imports: Rcpp
 LinkingTo: Rcpp
 URL: https://github.com/metrumresearchgroup/lastdose
 BugReports: https://github.com/metrumresearchgroup/lastdose/issues
-Suggests: testthat
+Suggests: testthat, withr
 Encoding: UTF-8
 LazyData: true
 RoxygenNote: 7.1.1
diff --git a/NEWS.md b/NEWS.md
index 159fbcb..f138f0f 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,15 @@
 # lastdose (development version)
 
+- Change default value for `include_tafd` to FALSE (#29)
+- Add option `lastdose.include_tafd` (#29)
+- Add option `lastdose.id_col`  (#27)
+- Add option `lastdose.time_units` (#27)
+- Search column names for candidate ID columns using `find_id_col()`; 
+  this function is unexported (#25)
+- Search column names for candidate time columns using `find_time_col()`;
+  this function is unexported (#25)
+- Handle missing values (`NA`) in the time column (#30)
+
 # lastdose 0.3.2
 
 - Fix bug where `II` column was not properly detected resulting in incorrect 
diff --git a/R/lastdose.R b/R/lastdose.R
index 2cf3829..a509bb2 100644
--- a/R/lastdose.R
+++ b/R/lastdose.R
@@ -15,11 +15,15 @@ NULL
 #' @param time_col character name for the `TIME` column; this could be time after
 #' first dose or time after first record or time relative to any origin; input
 #' may be `numeric` or `POSIXct` (e.g. `DATETIME`); if `POSIXct`, a numeric
-#' value will be calculated based on the value of `time_units`
+#' value will be calculated based on the value of `time_units`. The data frame
+#' will be searched for the first matching candidate time column using
+#' [find_time_col()].
 #' @param time_units for calculating time when the time column inherits
 #' `POSIXct`; you may use any value that is valid for [difftime()]
 #' @param id_col character name for the subject `ID` column; may be numeric
-#' or character; if character, a numeric value is derived
+#' or character; if character, a numeric value is derived. The data frame
+#' will be searched for the first matching candidate `ID` column using
+#' [find_id_col()].
 #' @param back_calc if `TRUE`, then the time before the first dose
 #' is calculated for records prior to the first dosing record when
 #' at least one dosing record is found in the data set.  Records before
@@ -41,13 +45,31 @@ NULL
 #' @param include_tafd `logical`; if `FALSE`, then `TAFD` data is not appended
 #' to the data set.  Only used for the [lastdose()] function.
 #'
+#' @section Options:
+#'
+#' These are options that can be set to customize `lastdose` behavior
+#' for the current context.  See `?options` for how to set an option.
+#'
+#' - `lastdose.time_units`: sets the default time unit that is used to calculate
+#'   relative times when the time column is represented as date-time data
+#'   (`POSIXct`)
+#' - `lastdose.id_col`: sets the default value for the `id_col` argument
+#'   to last dose; this identifies the column that is to be used to distinguish
+#'   individuals; the data in this column may be numeric or character
+#' - `lastdose.include_tafd`: sets default value for `include_tafd`; if `TRUE`
+#'   then the time since the first dose record (EVID 1 or EVID 4) in the data
+#'   set will be automatically appended to the output data frame when
+#'   calling `lastdose()`; `tafd` is always included when calling
+#'   `lastdose_df()` and `lastdose_list()`
+#'
 #' @details
 #'
-#' When calling [lastdose()] to modify the data frame, three columns will be
+#' When calling [lastdose()] to modify the data frame, two columns will be
 #' added (by default): `TAD` indicating the time after the most-recent dose,
-#' `TAFD` indicating the time after the first dose and `LDOS` indicating the
-#' amount of the most recent dose.  This default behavior can be modified with
-#' the `include_ldos` and `include_tafd` arguments.
+#' and `LDOS` indicating the amount of the most recent dose. `TAFD` indicating
+#' the time after the first dose record (EVID 1 or 4) can be added via the
+#' `include_tafd` argument and users can opt out from adding `LDOS` with the
+#' `include_ldos` argument.
 #'
 #' When calling [lastdose_list()] or [lastdose_df()], the respective items are
 #' accessible with `tad`,  `tafd`, and `ldos` (note the lower case form here to
@@ -102,8 +124,9 @@ NULL
 #'
 #'
 #' @export
-lastdose <- function(data,..., include_ldos = TRUE, include_tafd = TRUE) {
-  ans <- lastdose_list(data,...)
+lastdose <- function(data, ..., include_ldos = TRUE,
+                     include_tafd = getOption("lastdose.include_tafd", FALSE)) {
+  ans <- lastdose_list(data, ...)
   data[["TAD"]] <- ans[["tad"]]
   if(include_tafd) data[["TAFD"]] <- ans[["tafd"]]
   if(include_ldos) data[["LDOS"]] <- ans[["ldos"]]
@@ -113,40 +136,38 @@ lastdose <- function(data,..., include_ldos = TRUE, include_tafd = TRUE) {
 #' @rdname lastdose
 #' @export
 lastdose_list <- function(data,
-                          time_col = "TIME",
-                          time_units = NULL,
-                          id_col = "ID",
+                          time_col = find_time_col(data),
+                          time_units = getOption("lastdose.time_units", NULL),
+                          id_col = find_id_col(data),
                           fill = -99,
                           back_calc = TRUE,
                           addl_ties = c("obs_first", "dose_first"),
                           comments = find_comments(data)) {
-  if(length(comments)==1) {
+
+  if(length(comments) == 1) {
     comments <- rep(comments,nrow(data))
   }
-  if(!length(comments)==nrow(data)) {
-    stop("'comments' must be have length equal to the number of rows in 'data'",call.=FALSE)
+  if(length(comments) != nrow(data)) {
+    stop(
+      "'comments' must be have length equal to the number of rows in 'data'",
+      call. = FALSE
+    )
   }
   addl_ties <- match.arg(addl_ties)
   sort1 <- addl_ties == "obs_first"
   lower_names <- tolower(names(data))
-  wid <- match(id_col, names(data))
-  if(is.na(wid)) {
-    stop("did not find id column `", id_col, "` in `data`", call.=FALSE)
-  }
-  col_id <- data[[wid]]
-  if(is.character(col_id)) {
-    col_id <- match(col_id, unique(col_id))
-  }
-  if(!is.numeric(col_id)) {
-    stop("id column is required to be numeric", call.=FALSE)
-  }
   wtime <- match(time_col, names(data))
   if(is.na(wtime)) {
-    stop("did not find time column `", time_col, "` in `data`", call.=FALSE)
+    stop("did not find time column `", time_col, "` in `data`", call. = FALSE)
+  }
+  has_na_time <- anyNA(data[[wtime]])
+  if(has_na_time) {
+    na_time <- is.na(data[[wtime]])
+    data <- data[!na_time,, drop = FALSE]
   }
   col_time <- data[[wtime]]
   if(inherits(col_time, "POSIXct")) {
-    if(missing(time_units)) {
+    if(is.null(time_units)) {
       stop(
         "`time_units` is required when time column inherits `POSIXct`",
         call.=FALSE
@@ -164,6 +185,17 @@ lastdose_list <- function(data,
   if(!is.numeric(col_time)) {
     stop("time column is required to be numeric", call.=FALSE)
   }
+  wid <- match(id_col, names(data))
+  if(is.na(wid)) {
+    stop("did not find id column `", id_col, "` in `data`", call.=FALSE)
+  }
+  col_id <- data[[wid]]
+  if(is.character(col_id)) {
+    col_id <- match(col_id, unique(col_id))
+  }
+  if(!is.numeric(col_id)) {
+    stop("id column is required to be numeric", call.=FALSE)
+  }
   wamt <- match("amt", lower_names)
   if(is.na(wamt)) {
     stop("column AMT or amt is required in the data set", call.=FALSE)
@@ -214,19 +246,25 @@ lastdose_list <- function(data,
     sort1,
     comments
   )
+  if(has_na_time) {
+    re_order <- order(c(which(!na_time), which(na_time)))
+    for(j in seq_along(ans)) {
+      ans[[j]] <- ans[[j]][re_order]
+    }
+  }
   ans
 }
 
 #' @rdname lastdose
 #' @export
-lastdose_df <- function(data,...) {
-  ans <- lastdose_list(data,...)
+lastdose_df <- function(data, ...) {
+  ans <- lastdose_list(data, ...)
   data.frame(
     tad = ans[["tad"]],
     tafd = ans[["tafd"]],
     ldos = ans[["ldos"]],
-    stringsAsFactors=FALSE,check.names=FALSE,
-    fix.empty.names=FALSE, row.names=NULL
+    stringsAsFactors = FALSE, check.names = FALSE,
+    fix.empty.names = FALSE, row.names = NULL
   )
 }
 
@@ -260,11 +298,11 @@ lastdose_df <- function(data,...) {
 #'
 #'
 #' @export
-find_comments <- function(x,...) UseMethod("find_comments")
+find_comments <- function(x, ...) UseMethod("find_comments")
 #' @rdname find_comments
 #'
 #' @export
-find_comments.data.frame <- function(x,...) {
+find_comments.data.frame <- function(x, ...) {
   if(!inherits(x[["C"]], c("logical", "character"))) {
     if(exists("C", x)) {
       warning(
@@ -279,7 +317,7 @@ find_comments.data.frame <- function(x,...) {
 
 #' @rdname find_comments
 #' @export
-find_comments.character <- function(x,...) {
+find_comments.character <- function(x, ...) {
   !(is.na(x)|x=='.')
 }
 
@@ -288,3 +326,66 @@ find_comments.character <- function(x,...) {
 find_comments.logical <- function(x, ...) {
   x & !is.na(x)
 }
+
+#' Find TIME column
+#'
+#' Search data frame names for the first matching candidate TIME column name.
+#' See `details`.
+#'
+#' @param data a data.frame to search
+#' @details
+#' Column names will be searched against the following candidates
+#'
+#' - `TIME`
+#' - `DATETIME`
+#'
+#' The first the first candidate to be matched will be returned. If there
+#' are no matches, an error is generated.
+#'
+#' @examples
+#' data <- data.frame(A = 1, DATETIME = 2, TIME = 3, Z = 99)
+#' lastdose:::find_time_col(data)
+#'
+find_time_col <- function(data) {
+  stopifnot(is.data.frame(data))
+  ans <- intersect(c("TIME", "DATETIME"), names(data))
+  if(length(ans)==0) {
+    stop("could not find a TIME column in `data`", call. = FALSE)
+  }
+  ans[1]
+}
+
+#' Find ID column
+#'
+#' Search data frame names for the first matching candidate ID column name.
+#' See `details`.
+#'
+#' @param data a data.frame to search
+#' @details
+#' Column names will be searched against the following candidates
+#'
+#' - `getOption("lastdose.id_col")`
+#' - `ID`
+#' - `USUBJID`
+#' - `SUBJID`
+#' - `PTNO`
+#' - `SUBJ`
+#'
+#' The first the first candidate to be matched will be returned. If there
+#' are no matches, an error is generated.
+#'
+#' @examples
+#' data <- data.frame(A = 1, B = 2, PTNO = 3, ID = 4, Z = 99)
+#' lastdose:::find_id_col(data)
+#'
+find_id_col <- function(data) {
+  stopifnot(is.data.frame(data))
+  op <- getOption("lastdose.id_col", NULL)
+  can <- c(op, "ID", "USUBJID", "SUBJID", "PTNO", "SUBJ")
+  ans <- intersect(can, names(data))
+  if(length(ans)==0) {
+    stop("could not find a subject identifier column in `data`", call. = FALSE)
+  }
+  ans[1]
+}
+
diff --git a/man/find_id_col.Rd b/man/find_id_col.Rd
new file mode 100644
index 0000000..f861c33
--- /dev/null
+++ b/man/find_id_col.Rd
@@ -0,0 +1,34 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/lastdose.R
+\name{find_id_col}
+\alias{find_id_col}
+\title{Find ID column}
+\usage{
+find_id_col(data)
+}
+\arguments{
+\item{data}{a data.frame to search}
+}
+\description{
+Search data frame names for the first matching candidate ID column name.
+See \code{details}.
+}
+\details{
+Column names will be searched against the following candidates
+\itemize{
+\item \code{getOption("lastdose.id_col")}
+\item \code{ID}
+\item \code{USUBJID}
+\item \code{SUBJID}
+\item \code{PTNO}
+\item \code{SUBJ}
+}
+
+The first the first candidate to be matched will be returned. If there
+are no matches, an error is generated.
+}
+\examples{
+data <- data.frame(A = 1, B = 2, PTNO = 3, ID = 4, Z = 99)
+lastdose:::find_id_col(data)
+
+}
diff --git a/man/find_time_col.Rd b/man/find_time_col.Rd
new file mode 100644
index 0000000..d644932
--- /dev/null
+++ b/man/find_time_col.Rd
@@ -0,0 +1,30 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/lastdose.R
+\name{find_time_col}
+\alias{find_time_col}
+\title{Find TIME column}
+\usage{
+find_time_col(data)
+}
+\arguments{
+\item{data}{a data.frame to search}
+}
+\description{
+Search data frame names for the first matching candidate TIME column name.
+See \code{details}.
+}
+\details{
+Column names will be searched against the following candidates
+\itemize{
+\item \code{TIME}
+\item \code{DATETIME}
+}
+
+The first the first candidate to be matched will be returned. If there
+are no matches, an error is generated.
+}
+\examples{
+data <- data.frame(A = 1, DATETIME = 2, TIME = 3, Z = 99)
+lastdose:::find_time_col(data)
+
+}
diff --git a/man/lastdose.Rd b/man/lastdose.Rd
index 1586d96..11c6dd0 100644
--- a/man/lastdose.Rd
+++ b/man/lastdose.Rd
@@ -6,13 +6,18 @@
 \alias{lastdose_df}
 \title{Calculate last dose amount and times since previous doses}
 \usage{
-lastdose(data, ..., include_ldos = TRUE, include_tafd = TRUE)
+lastdose(
+  data,
+  ...,
+  include_ldos = TRUE,
+  include_tafd = getOption("lastdose.include_tafd", FALSE)
+)
 
 lastdose_list(
   data,
-  time_col = "TIME",
-  time_units = NULL,
-  id_col = "ID",
+  time_col = find_time_col(data),
+  time_units = getOption("lastdose.time_units", NULL),
+  id_col = find_id_col(data),
   fill = -99,
   back_calc = TRUE,
   addl_ties = c("obs_first", "dose_first"),
@@ -35,13 +40,17 @@ to the data set.  Only used for the \code{\link[=lastdose]{lastdose()}} function
 \item{time_col}{character name for the \code{TIME} column; this could be time after
 first dose or time after first record or time relative to any origin; input
 may be \code{numeric} or \code{POSIXct} (e.g. \code{DATETIME}); if \code{POSIXct}, a numeric
-value will be calculated based on the value of \code{time_units}}
+value will be calculated based on the value of \code{time_units}. The data frame
+will be searched for the first matching candidate time column using
+\code{\link[=find_time_col]{find_time_col()}}.}
 
 \item{time_units}{for calculating time when the time column inherits
 \code{POSIXct}; you may use any value that is valid for \code{\link[=difftime]{difftime()}}}
 
 \item{id_col}{character name for the subject \code{ID} column; may be numeric
-or character; if character, a numeric value is derived}
+or character; if character, a numeric value is derived. The data frame
+will be searched for the first matching candidate \code{ID} column using
+\code{\link[=find_id_col]{find_id_col()}}.}
 
 \item{fill}{the value for \code{TAD} and \code{TAFD} that is used for records when no
 doses are found for an individual or when \code{back_calc} is \code{FALSE}.}
@@ -70,11 +79,12 @@ to add (or potentially replace) columns to the input data frame;
 as either \code{list} or \code{data.frame} format without modifying the input data.
 }
 \details{
-When calling \code{\link[=lastdose]{lastdose()}} to modify the data frame, three columns will be
+When calling \code{\link[=lastdose]{lastdose()}} to modify the data frame, two columns will be
 added (by default): \code{TAD} indicating the time after the most-recent dose,
-\code{TAFD} indicating the time after the first dose and \code{LDOS} indicating the
-amount of the most recent dose.  This default behavior can be modified with
-the \code{include_ldos} and \code{include_tafd} arguments.
+and \code{LDOS} indicating the amount of the most recent dose. \code{TAFD} indicating
+the time after the first dose record (EVID 1 or 4) can be added via the
+\code{include_tafd} argument and users can opt out from adding \code{LDOS} with the
+\code{include_ldos} argument.
 
 When calling \code{\link[=lastdose_list]{lastdose_list()}} or \code{\link[=lastdose_df]{lastdose_df()}}, the respective items are
 accessible with \code{tad},  \code{tafd}, and \code{ldos} (note the lower case form here to
@@ -119,6 +129,26 @@ dosing and both an an additional dose and an observation happen at 24 hours,
 \code{obs_first} will set the observation \code{TAD} to 24 and \code{dose_first} will set
 the observation \code{TAD} to 0.
 }
+\section{Options}{
+
+
+These are options that can be set to customize \code{lastdose} behavior
+for the current context.  See \code{?options} for how to set an option.
+\itemize{
+\item \code{lastdose.time_units}: sets the default time unit that is used to calculate
+relative times when the time column is represented as date-time data
+(\code{POSIXct})
+\item \code{lastdose.id_col}: sets the default value for the \code{id_col} argument
+to last dose; this identifies the column that is to be used to distinguish
+individuals; the data in this column may be numeric or character
+\item \code{lastdose.include_tafd}: sets default value for \code{include_tafd}; if \code{TRUE}
+then the time since the first dose record (EVID 1 or EVID 4) in the data
+set will be automatically appended to the output data frame when
+calling \code{lastdose()}; \code{tafd} is always included when calling
+\code{lastdose_df()} and \code{lastdose_list()}
+}
+}
+
 \examples{
 file <- system.file("csv/data1.csv", package="lastdose")
 
diff --git a/src/lastdose.cpp b/src/lastdose.cpp
index fd5a099..23ab7f3 100644
--- a/src/lastdose.cpp
+++ b/src/lastdose.cpp
@@ -103,7 +103,7 @@ Rcpp::List lastdose_impl(Rcpp::NumericVector id,
     double last_time = -1E9;
     for(int j = idstart[i]; j <= idend[i]; ++j) {
       // If time is missing
-      if(Rcpp::NumericVector::is_na(time[j])) {
+      if(isna(time[j])) {
         tad[crow] = NA_REAL;
         ldos[crow] = NA_REAL;
         ++crow;
@@ -121,7 +121,7 @@ Rcpp::List lastdose_impl(Rcpp::NumericVector id,
       }
       // Deal with missing dose
       bool missing_amt = Rcpp::NumericVector::is_na(amt[j]);
-      if(is_dose(evid[j],comment[j])) {
+      if(is_dose(evid[j], comment[j])) {
         if(!found_dose) {
           found_dose = true;
           tofd[i] = time[j];
diff --git a/tests/testthat/test-lastdose.R b/tests/testthat/test-lastdose.R
index df912b9..083715e 100644
--- a/tests/testthat/test-lastdose.R
+++ b/tests/testthat/test-lastdose.R
@@ -24,6 +24,8 @@ test_that("doses at time zero", {
 
 test_that("time after first dose", {
   x <- lastdose(set4)
+  expect_false(exists("TAFD", x))
+  x <- lastdose(set4, include_tafd = TRUE)
   expect_true(exists("TAFD", x))
   dose_rows <- which(set4$evid==1)
   time_of_first_dose <- set4$TIME[dose_rows[1]]
@@ -119,6 +121,53 @@ test_that("error for missing values in ID,evid,ii,addl", {
   }
 })
 
+test_that("handle missing values in time colunn", {
+  dd <- df[df$set==4,]
+  dd$TIME <- as.numeric(dd$TIME)
+  dd0 <- dd
+  set.seed(1010)
+  i <- sample(seq(nrow(dd)), size = 18)
+  i <- i[!dd$evid[i]==1]
+  dd$TIME[i] <- NA_real_
+  ans1 <- lastdose(dd)
+  ans2 <- lastdose(dd0)
+  w <- setdiff(seq(nrow(dd)),i)
+  for(col in names(ans1)) {
+    expect_identical(ans1[w,col], ans2[w,col])
+  }
+  ans3 <- ans1[i,]
+  expect_true(all(is.na(ans3$TAD)))
+  expect_true(all(is.na(ans3$LDOS)))
+  expect_true(all(is.na(ans3$TAFD)))
+  ans4 <- ans1[w,]
+  expect_false(any(is.na(ans4$TAD)))
+  expect_false(any(is.na(ans4$LDOS)))
+  expect_false(any(is.na(ans4$TAFD)))
+
+  file <- system.file("csv", "data_big.RDS", package = "lastdose")
+  data <- readRDS(file)
+  set.seed(21032)
+  x <- sample(seq(nrow(data)), 1000)
+  x <- x[data$EVID[x] ==0]
+  data2 <- data
+  data2$TIME[x] <- NA_real_
+  out1 <- lastdose(data,  include_tafd = TRUE)
+  out2 <- lastdose(data2, include_tafd = TRUE)
+  ans <- as.numeric(c(0,  length(x)))
+  smr <- function(x,y) {
+    x <- x-y
+    c(sum(x, na.rm = TRUE), sum(is.na(x)))
+  }
+  a <- smr(out1$TAD,  out2$TAD)
+  b <- smr(out1$TIME, out2$TIME)
+  c <- smr(out1$LDOS, out2$LDOS)
+  d <- smr(out1$TAFD, out2$TAFD)
+  expect_identical(a, ans)
+  expect_identical(b, ans)
+  expect_identical(c, ans)
+  expect_identical(d, ans)
+})
+
 test_that("NA amt is error for dosing record, ok otherwise", {
   dd <- set1
   dd$amt[5] <- NA_real_
@@ -162,23 +211,64 @@ test_that("undefined behavior when checking ADDL and II issue-11", {
 test_that("user-named time and id columns", {
   d1 <- subset(set1, ID==1)
   d2 <- d1
-  d2$TAFD <- d2$TIME
+  d2$xTAFD <- d2$TIME
   d2$TIME <- NULL
   expect_identical(
     lastdose_df(d1),
-    lastdose_df(d2, time_col = "TAFD")
+    lastdose_df(d2, time_col = "xTAFD")
   )
   expect_error(lastdose(d2), msg = "did not find time column")
   d2 <- d1
-  d2$USUBJID <- "A"
+  d2$xUSUBJID <- "A"
   d2$ID <- NULL
   expect_identical(
     lastdose_df(d1),
-    lastdose_df(d2, id_col = "USUBJID")
+    lastdose_df(d2, id_col = "xUSUBJID")
   )
+  if(requireNamespace("withr")) {
+    expect_identical(
+      lastdose_df(d2, id_col = "xUSUBJID"),
+      withr::with_options(
+        list(lastdose.id_col = "xUSUBJID"),
+        lastdose_df(d2)
+      )
+    )
+  }
   expect_error(lastdose(d2))
 })
 
+test_that("find time column from candidate list", {
+  dd <- subset(set1, ID==1)
+  time <- dd$TIME
+  dd$TIME <- NULL
+  tr <- c("TIME", "DATETIME")
+  for(col in tr) {
+    dd[[col]] <- time
+    expect_is(lastdose(dd), "data.frame")
+    dd[[col]] <- NULL
+  }
+})
+
+test_that("find ID column from candidate list", {
+  dd <- subset(set1, ID==1)[1:3,]
+  ID <- dd$ID
+  dd$ID <- NULL
+  tr <- c("ID", "USUBJID", "SUBJID", "PTNO", "SUBJ")
+  for(col in tr) {
+    dd[[col]] <- ID
+    expect_is(lastdose(dd), "data.frame")
+    dd[[col]] <- NULL
+  }
+  if(requireNamespace("withr")) {
+    dd[["i_d"]] <- ID
+    ans1 <- withr::with_options(
+      list(lastdose.id_col = "i_d"),
+      lastdose(dd)
+    )
+    expect_is(ans1, "data.frame")
+  }
+})
+
 test_that("POSIXct datetime is converted to numeric time", {
   d1 <- subset(set1, ID <= 2)
   d2 <- d1
@@ -192,6 +282,14 @@ test_that("POSIXct datetime is converted to numeric time", {
   expect_error(
     lastdose(d1, time_units = "seconds")
   )
+  if(requireNamespace("withr")) {
+    ans1 <- withr::with_options(
+      list(lastdose.time_units = "hours"),
+      lastdose(d1)
+    )
+    ans2 <- lastdose(d1, time_units = "hours")
+    expect_identical(ans1, ans2)
+  }
 })
 
 test_that("logical comment column is ok", {