t1-t2 only gains difftime class optionally

NEWS.md

@@ -12,6 +12,10 @@
 
 3. options `"datatable.old.matrix.autoname"` is now `FALSE` by default, meaning `names(data.table(x=1, cbind(1)))` is now `c("x", "V2")`. Toggle the option to retain the old behavior for now; future releases will work to remove this possibility. See the release notes for 1.18.0, item 1 under `NOTE OF INTENDED FUTURE POTENTIAL BREAKING CHANGES`.
 
+### NOTICE OF INTENDED FUTURE POTENTIAL BREAKING CHANGES  
+
+1. `t1 - t2`, where both `t1` and `t2` are `IDate`, will eventually have class `difftime`, consistent with the case where `t1` and `t2` are both `Date`. You can activate the new behavior by setting `options(datatable.old.diff.time = FALSE)`. See point 17 under Bug Fixes for more context.
+
 ### NEW FEATURES
 
 1. `nafill()`, `setnafill()` extended to work on logical and factor vectors (part of [#3992](https://github.com/Rdatatable/data.table/issues/3992)). Includes support for `Date`, `IDate`, `POSIXct`, etc. `nafill()` works for character vectors, but not yet `setnafill()`. Thanks @jangorecki for the request and @jangorecki and @MichaelChirico for the PRs.

R/IDateTime.R

@@ -128,8 +128,10 @@ chooseOpsMethod.IDate = function(x, y, mx, my, cl, reverse) inherits(y, "Date")
   }
   ans = as.integer(unclass(e1) - unclass(e2))
   if (inherits(e2, "Date")) {
-    setattr(ans, "class", "difftime")
-    setattr(ans, "units", "days")
+    if (!isTRUE(getOption("datatable.old.diff.idate"))) {
+      setattr(ans, "class", "difftime")
+      setattr(ans, "units", "days")
+    }
   } else {
     setattr(ans, "class", c("IDate", "Date"))
   }

R/onLoad.R

@@ -98,7 +98,8 @@
     datatable.auto.index=TRUE,          # DT[col=="val"] to auto add index so 2nd time faster
     datatable.use.index=TRUE,           # global switch to address #1422
     datatable.prettyprint.char=NULL,    # FR #1091
-    datatable.old.matrix.autoname=FALSE # #7145: how data.table(x=1, matrix(1)) is auto-named set to change
+    datatable.old.matrix.autoname=FALSE,# #7145: how data.table(x=1, matrix(1)) is auto-named set to change
+    datatable.old.diff.idate=TRUE       # whether <IDate>-<IDate> gets difftime class set to change
   )
   opts = opts[!names(opts) %chin% names(options())]
   options(opts)

inst/tests/tests.Rraw

@@ -10406,7 +10406,7 @@ test(1673.1, TT + 4L, as.IDate("2016-04-29"))
 test(1673.2, TT + 4, as.IDate("2016-04-29"))
 test(1673.3, TT - 3, as.IDate("2016-04-22"))
 test(1673.4, TT - 3L, as.IDate("2016-04-22"))
-test(1673.5, all.equal(as.IDate("2016-04-28") - as.IDate("2016-04-20"), as.difftime(8, units='days')))
+test(1673.5, options=c(datatable.old.diff.idate=FALSE), all.equal(as.IDate("2016-04-28") - as.IDate("2016-04-20"), as.difftime(8, units='days')))
 
 
 # test for radix integer order when MAXINT is present AND decreasing=TRUE AND na.last=FALSE
@@ -21676,15 +21676,19 @@ test(2335.5, isoyear("2019-12-30"), 2020L)
 test(2335.6, isoyear(as.Date("2019-12-30")), 2020L)
 
 # t1-t2 for Date/IDate should be consistent, modulo storage mode #4979
+t1 = as.IDate("2025-07-01")
+t2 = as.IDate("2025-06-01")
+test(2336.01, options=c(datatable.old.diff.idate=FALSE), all.equal(as.Date(t1) - as.Date(t2), t1 - t2))
+test(2336.02, options=c(datatable.old.diff.idate=FALSE), all.equal(as.Date(t2) - as.Date(t1), t2 - t1))
+test(2336.03, options=c(datatable.old.diff.idate=FALSE), all.equal(as.Date(t1) - t2, t1 - t2))
+test(2336.04, options=c(datatable.old.diff.idate=FALSE), all.equal(as.Date(t2) - t1, t2 - t1))
 if (base::getRversion() >= "4.3.0") { ## follow up of #7213, see #7321
-  t1 = as.IDate("2025-07-01")
-  t2 = as.IDate("2025-06-01")
-  test(2336.1, all.equal(as.Date(t1) - as.Date(t2), t1 - t2))
-  test(2336.2, all.equal(as.Date(t2) - as.Date(t1), t2 - t1))
-  test(2336.3, all.equal(as.Date(t1) - t2, t1 - t2))
-  test(2336.4, all.equal(as.Date(t2) - t1, t2 - t1))
-  test(2336.5, all.equal(t1 - as.Date(t2), t1 - t2))
-  test(2336.6, all.equal(t2 - as.Date(t1), t2 - t1))
+  test(2336.05, all.equal(t1 - as.Date(t2), t1 - t2))
+  test(2336.06, all.equal(t2 - as.Date(t1), t2 - t1))
+  test(2336.07, all.equal(as.numeric(as.Date(t1) - as.Date(t2)), as.numeric(t1 - t2)))
+  test(2336.08, all.equal(as.numeric(as.Date(t2) - as.Date(t1)), as.numeric(t2 - t1)))
+  test(2336.09, all.equal(as.numeric(as.Date(t1) - t2), as.numeric(t1 - t2)))
+  test(2336.10, all.equal(as.numeric(as.Date(t2) - t1), as.numeric(t2 - t1)))
 }
 
 # fwrite: allow dec=',' with single column, #7227

man/data.table-options.Rd

@@ -114,6 +114,10 @@
       expressions like \code{data.table(x=1, cbind(1))} will be named. When \code{TRUE}, it will be named
       \code{V1}, otherwise it will be named \code{V2}.
     }
+    \item{\code{datatable.old.diff.idate}}{Logical, default \code{TRUE}. Governs whether \code{t1 - t2},
+      where \code{t1} and \code{t2} are both class \code{IDate}, gains the S3 class \code{difftime}.
+      When \code{TRUE}, this class is assigned (with \code{units="days"}).
+    }
   }
 }