From ae63c5de764c6e01c1f875ae03a8c04f8b6f22c4 Mon Sep 17 00:00:00 2001
From: Ed Burn <9583964+edward-burn@users.noreply.github.com>
Date: Sat, 6 Dec 2025 12:45:59 +0000
Subject: [PATCH] use DATEDIFF_BIG on sql server

---
 NEWS.md                             | 1 +
 R/backend-mssql.R                   | 8 ++++----
 tests/testthat/test-backend-mssql.R | 6 +++---
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index 6e91fe5ff..db13e1ca6 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,6 @@
 # dbplyr (development version)
 
+* SQL Server uses `DATEDIFF_BIG` instead of `DATEDIFF` to work regardless of data size (@edward-burn, #1666).
 * SQL Server: `if`/`ifelse()`, and `if_else()` now use `CASE WHEN` instead of `IIF`. This ensures the handling of `NULL`s matches the R's `NA` handling rules (#1569). 
 * `if_else()` uses simpler translation for `missing` (#1573).
 * New translations for stringr function `str_ilike()` for Postgres, Redshift, and Snowflake (@edward-burn, #1628).
diff --git a/R/backend-mssql.R b/R/backend-mssql.R
index 93d15a60f..96a78d12e 100644
--- a/R/backend-mssql.R
+++ b/R/backend-mssql.R
@@ -414,15 +414,15 @@ simulate_mssql <- function(version = "15.0") {
         check_dots_empty()
         check_unsupported_arg(precision, allowed = "day")
         check_unsupported_arg(n, allowed = 1L)
-
-        sql_expr(DATEDIFF(DAY, !!start, !!end))
+        # https://learn.microsoft.com/en-us/sql/t-sql/functions/datediff-big-transact-sql?view=sql-server-ver17
+        sql_expr(DATEDIFF_BIG(DAY, !!start, !!end))
       },
 
       difftime = function(time1, time2, tz, units = "days") {
         check_unsupported_arg(tz)
         check_unsupported_arg(units, allowed = "days")
 
-        sql_expr(DATEDIFF(DAY, !!time2, !!time1))
+        sql_expr(DATEDIFF_BIG(DAY, !!time2, !!time1))
       }
     )
 
@@ -723,7 +723,7 @@ utils::globalVariables(c(
   "DATE",
   "DATEADD",
   "DATEFROMPARTS",
-  "DATEDIFF",
+  "DATEDIFF_BIG",
   "DATENAME",
   "DATEPART",
   "IIF",
diff --git a/tests/testthat/test-backend-mssql.R b/tests/testthat/test-backend-mssql.R
index f3fc6a3a7..10f399355 100644
--- a/tests/testthat/test-backend-mssql.R
+++ b/tests/testthat/test-backend-mssql.R
@@ -189,7 +189,7 @@ test_that("custom clock functions translated correctly", {
   expect_translation(
     con,
     date_count_between(date_column_1, date_column_2, "day"),
-    "DATEDIFF(DAY, `date_column_1`, `date_column_2`)"
+    "DATEDIFF_BIG(DAY, `date_column_1`, `date_column_2`)"
   )
   expect_snapshot(
     error = TRUE,
@@ -217,12 +217,12 @@ test_that("difftime is translated correctly", {
   expect_translation(
     con,
     difftime(start_date, end_date, units = "days"),
-    "DATEDIFF(DAY, `end_date`, `start_date`)"
+    "DATEDIFF_BIG(DAY, `end_date`, `start_date`)"
   )
   expect_translation(
     con,
     difftime(start_date, end_date),
-    "DATEDIFF(DAY, `end_date`, `start_date`)"
+    "DATEDIFF_BIG(DAY, `end_date`, `start_date`)"
   )
 
   expect_snapshot(