Add Date method for impute_below

[njtierney]

shadow_shift and therefore impute_below don't handle date objects:

dat_date <- data.frame(date = as.POSIXct(c(111, 
                                           112,
                                           NA,
                                           NA, 
                                           108,
                                           150,
                                           160),
                                         origin = "1970-01-01"))

# as.numeric(dat_date$date)

naniar::shadow_shift(dat_date$date)
#> Error: shadow_shift does not know how to deal with data of class POSIXctPOSIXt please check your input is more than length one

Created on 2018-05-03 by the reprex package (v0.2.0).

This should be able to be fixed by doing some conversion to a numeric object, then sending it back as a whole integer.

[njtierney]

One thing to keep in mind here is:

• How do we find out what the original date origin was?
• Just how many date types are there?

[njtierney]

It seems reasonable to convert the date to numeric and then coerce back to a date

[njtierney]

There are three extra methods now:

• POSIXct
• POSIXlt
• Date