diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index d8fad8cc2..f1c5e017e 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -421,14 +421,17 @@ test.list <- atime::atime_test_list(
     expr = {
       ns = environment(data.table::as.IDate)
       s3_table = get(".__S3MethodsTable__.", envir = baseenv())
-      if (exists("chooseOpsMethod.IDate", envir = s3_table)) {
-        rm("chooseOpsMethod.IDate", envir = s3_table)
-      }
-      if (exists("chooseOpsMethod.IDate", envir = ns, inherits = FALSE)) {
-        base::registerS3method(
-          "chooseOpsMethod", "IDate",
-          get("chooseOpsMethod.IDate", envir = ns),
-          envir = ns)
+      s3_generics = c("chooseOpsMethod.IDate" = "chooseOpsMethod", "-.IDate" = "-")
+      for (s3_method in names(s3_generics)) {
+        if (exists(s3_method, envir = s3_table, inherits = FALSE)) {
+          rm(list = s3_method, envir = s3_table)
+        }
+        if (exists(s3_method, envir = ns, inherits = FALSE)) {
+          base::registerS3method(
+            s3_generics[[s3_method]], "IDate",
+            get(s3_method, envir = ns, inherits = FALSE),
+            envir = ns)
+        }
       }
       outer(short_date, data.table::as.IDate(long_date), `-`)
     }),
diff --git a/NEWS.md b/NEWS.md
index 025e9a578..6693cee54 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -62,6 +62,8 @@
 
 13. `rbindlist()` (and therefore the `rbind()` method for `data.table`s) no longer raises an error upon encountering more than approximately 50000 columns in a list entry, [#7793](https://github.com/Rdatatable/data.table/issues/7793). The bug was introduced in `data.table` version 1.18.2.1. Thanks to @rickhelmus for the report and @aitap for the fix.
 
+14. Subtracting an `IDate` from a `Date` is fast again by avoiding unnecessary conversion to `POSIXlt`/`POSIXct`, [#7825](https://github.com/Rdatatable/data.table/issues/7825). Thanks @gilesheywood for the report and @ben-schwen for the fix.
+
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).
diff --git a/R/IDateTime.R b/R/IDateTime.R
index 99e6f64b2..7f2192671 100644
--- a/R/IDateTime.R
+++ b/R/IDateTime.R
@@ -111,7 +111,14 @@ chooseOpsMethod.IDate = function(x, y, mx, my, cl, reverse) inherits(y, "Date")
 
 `-.IDate` = function(e1, e2) {
   if (!inherits(e1, "IDate")) {
-    if (inherits(e1, 'Date')) return(base::`-.Date`(e1, e2))
+    if (inherits(e1, "Date")) {
+      if (!inherits(e2, "Date")) return(base::`-.Date`(e1, e2))
+      #7825 avoid base::`-.Date` to avoid conversion from IDate to POSIXlt/POSIXct
+      ans = unclass(e1) - unclass(e2)
+      setattr(ans, "class", "difftime")
+      setattr(ans, "units", "days")
+      return(ans)
+    }
     stopf("can only subtract from \"IDate\" objects")
   }
   if (storage.mode(e1) != "integer")