t1-t2 only gains difftime class optionally, for now

Author: MichaelChirico

NEWS.md

@@ -8,6 +8,8 @@
 
 1. `data.table(x=1, <expr>)`, where `<expr>` is an expression resulting in a 1-column matrix without column names, will eventually have names `x` and `V2`, not `x` and `V1`, consistent with `data.table(x=1, <expr>)` where `<expr>` results in an atomic vector, for example `data.table(x=1, cbind(1))` and `data.table(x=1, 1)` will both have columns named `x` and `V2`. In this release, the matrix case continues to be named `V1`, but the new behavior can be activated by setting `options(datatable.old.matrix.autoname)` to `FALSE`. See point 5 under Bug Fixes for more context; this change will provide more internal consistency as well as more consistency with `data.frame()`.
 
+2. `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`. See point 17 under Bug Fixes for more context.
+
 ### NEW FEATURES
 
 1. New `sort_by()` method for data.tables, [#6662](https://github.com/Rdatatable/data.table/issues/6662). It uses `forder()` to improve upon the data.frame method and also matches `DT[order(...)]` behavior with respect to locale. Thanks @rikivillalba for the suggestion and PR.

R/IDateTime.R

@@ -130,8 +130,10 @@ 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.date"))) {
+      setattr(ans, "class", "difftime")
+      setattr(ans, "units", "days")
+    }
   } else {
     setattr(ans, "class", c("IDate", "Date"))
   }

R/onLoad.R

@@ -92,7 +92,8 @@
     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=TRUE  # #7145: how data.table(x=1, matrix(1)) is auto-named set to change
+    datatable.old.matrix.autoname=TRUE, # #7145: how data.table(x=1, matrix(1)) is auto-named set to change
+    datatable.old.diff.date=TRUE        # whether <IDate>-<IDate> gets difftime class set to change
   )
   opts = opts[!names(opts) %chin% names(options())]
   options(opts)