diff --git a/tool/parse.cpp b/tool/parse.cpp
index b2fe84d..a08e8ff 100644
--- a/tool/parse.cpp
+++ b/tool/parse.cpp
@@ -60,7 +60,7 @@ std::string supported_duration_suffixes()
     return "\"ns\", \"us\", \"ms\", \"s\", \"m\", \"h\"";
 }
 
-bool parse_duration(const char* name, const std::string& in, uint64_t& result)
+bool parse_duration(const char* name, const std::string& in, int64_t& result)
 {
     uint64_t multiplier = 0;
 
@@ -105,7 +105,7 @@ bool parse_duration(const char* name, const std::string& in, uint64_t& result)
         return false;
     }
 
-    result = (uint64_t)number * multiplier;
+    result = (int64_t)number * multiplier;
     return true;
 }
 
@@ -113,12 +113,12 @@ bool parse_duration(const char* name,
     const std::string& in,
     google::protobuf::Duration& out)
 {
-    uint64_t nanoseconds = 0;
+    int64_t nanoseconds = 0;
     if (!parse_duration(name, in, nanoseconds)) {
         return false;
     }
 
-    out = google::protobuf::util::TimeUtil::NanosecondsToDuration((int64_t)nanoseconds);
+    out = google::protobuf::util::TimeUtil::NanosecondsToDuration(nanoseconds);
     return true;
 }
 
diff --git a/tool/parse.hpp b/tool/parse.hpp
index 17200f1..7b17cdd 100644
--- a/tool/parse.hpp
+++ b/tool/parse.hpp
@@ -23,7 +23,7 @@ bool parse_index(const std::string& in, uint32_t& out);
 
 std::string supported_duration_suffixes();
 
-bool parse_duration(const char* name, const std::string& in, uint64_t& out);
+bool parse_duration(const char* name, const std::string& in, int64_t& out);
 
 bool parse_duration(const char* name,
     const std::string& in,