diff --git a/NEWS.md b/NEWS.md
index e7acd1146..df98eec28 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).
 * `na_matches = "na"` now works correctly with inequality and overlap joins, preserving the comparison operator instead of converting to equality (#1505).
 * `copy_inline()` now works with blob columns (#1515).
 * `tbl_sql()` is now (soft) deprecated. It hasn't been required in a while. 
diff --git a/R/backend-mssql.R b/R/backend-mssql.R
index eaf93915c..4f572de20 100644
--- a/R/backend-mssql.R
+++ b/R/backend-mssql.R
@@ -417,15 +417,15 @@ simulate_mssql <- function(version = "15.0") {
         check_dots_empty()
         check_unsupported_arg(precision, allowed = "day")
         check_unsupported_arg(n, allowed = 1L)
-
-        sql_glue("DATEDIFF(DAY, {start}, {end})")
+        # https://learn.microsoft.com/en-us/sql/t-sql/functions/datediff-big-transact-sql?view=sql-server-ver17
+        sql_glue("DATEDIFF_BIG(DAY, {start}, {end})")
       },
 
       difftime = function(time1, time2, tz, units = "days") {
         check_unsupported_arg(tz)
         check_unsupported_arg(units, allowed = "days")
 
-        sql_glue("DATEDIFF(DAY, {time2}, {time1})")
+        sql_glue("DATEDIFF_BIG(DAY, {time2}, {time1})")
       }
     )
 
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(