Improve strptime wrapper warning message

NEWS.md

@@ -287,6 +287,8 @@
 
 16. Rolling functions (`?froll`) coerce `logical` input to `numeric` (instead of failing) to mimic the behavior of `integer` input.
 
+17. The warning message when using `strptime` in `j` has been improved, [#2068](https://github.com/Rdatatable/data.table/issues/2068). Thanks to @tdhock for the report.
+
 
 ### Changes in [v1.12.2](https://github.com/Rdatatable/data.table/milestone/14?closed=1)  (07 Apr 2019)
 

R/data.table.R

@@ -1155,7 +1155,7 @@ replace_order = function(isub, verbose, env) {
   SDenv = new.env(parent=parent.frame())
   # taking care of warnings for posixlt type, #646
   SDenv$strptime = function(x, ...) {
-    warning("POSIXlt column type detected and converted to POSIXct. We do not recommend use of POSIXlt at all because it uses 40 bytes to store one date. Use as.POSIXct to avoid this warning.")
+    warning("strptime() usage detected and wrapped with as.POSIXct(). This is to minimize the chance of assigning POSIXlt columns, which use 40+ bytes to store one date (versus 8 for POSIXct). Use as.POSIXct() (which will call strptime() as needed internally) to avoid this warning.")
     as.POSIXct(base::strptime(x, ...))
   }
 

inst/tests/tests.Rraw

@@ -8244,7 +8244,7 @@ ll = list(a=as.POSIXlt("2015-01-01", tz='UTC'), b=1:5)
 test(1612.1, as.data.table(ll), data.table(a=as.POSIXct("2015-01-01", tz='UTC'), b=1:5), warning="POSIXlt column type detected")
 dt = data.table(d1="1984-03-17")
 ans = data.table(d1="1984-03-17", d2=as.POSIXct("1984-03-17", tz='UTC'))
-test(1612.2, dt[, d2 := strptime(d1, "%Y-%m-%d", tz='UTC')], ans, warning="POSIXlt column type detected and converted")
+test(1612.2, dt[, d2 := strptime(d1, "%Y-%m-%d", tz='UTC')], ans, warning="strptime() usage detected and wrapped with as.POSIXct()")
 ll = list(a=as.POSIXlt("2015-01-01"), b=2L)
 test(1612.3, setDT(ll), error="Column 1 is of POSIXlt type")