diff --git a/frontend/src/commonMain/resources/std/temporal/temporal.temper.md b/frontend/src/commonMain/resources/std/temporal/temporal.temper.md
index 6755d9c32..995461d4a 100644
--- a/frontend/src/commonMain/resources/std/temporal/temporal.temper.md
+++ b/frontend/src/commonMain/resources/std/temporal/temporal.temper.md
@@ -219,14 +219,19 @@ Here's just enough of a Date type to get us started.
        */
       @connected
       public get dayOfWeek(): Int {
-        // Gauss's method.
+        // Gauss's method: January first of year Y is weekday
+        // (1 + 5((Y-1) mod 4) + 4((Y-1) mod 100) + 6((Y-1) mod 400)) mod 7,
+        // with Sunday 0. It is Y - 1 that splits into century and year:
+        // splitting Y instead is wrong for every year ending in 00 that
+        // is not a multiple of 400, such as 1900 and 2100.
+        // `%` keeps the dividend's sign, so each remainder adds its
+        // divisor and takes it again, to stay in range for years before 1.
         let y = year;
-        let c = if (y >= 0) { y / 100 } else { -(-y / 100) };
-        let yy = y - (c * 100);
-        // See note below about avoiding negative modulus to see why
-        // some of the offsets differ from Wikipedia's rendering of
-        // Gauss's formula.
-        let janFirst = (8 + 5*((yy + 3) % 4) + 3*(yy - 1) + 5*(c % 4)) % 7;
+        let before = y - 1;
+        let mod4 = ((before % 4) + 4) % 4;
+        let mod100 = ((before % 100) + 100) % 100;
+        let mod400 = ((before % 400) + 400) % 400;
+        let janFirst = (1 + 5 * mod4 + 4 * mod100 + 6 * mod400) % 7;
         let table = if (isLeapYear(y)) {
           dayOfWeekLookupTableLeapy
         } else {
diff --git a/functional-test-suite/src/commonMain/resources/types/date/date.temper.md b/functional-test-suite/src/commonMain/resources/types/date/date.temper.md
index 3bf436e9a..0cde50130 100644
--- a/functional-test-suite/src/commonMain/resources/types/date/date.temper.md
+++ b/functional-test-suite/src/commonMain/resources/types/date/date.temper.md
@@ -78,6 +78,25 @@ ISO 8601 yonks: 0012-11-10
 yonksAgo.year = 12
 ```
 
+The weekday of a year ending in 00 needs the right century. A
+January first or March first of 1900 or 2100 lands on a different day
+than in 2000, since 2000 is a leap year and the others are not.
+
+    for (let y of [1700, 1800, 1900, 2000, 2100, 2400]) {
+      console.log(
+        "${y}: Jan 1 weekday ${new Date(y, 1, 1).dayOfWeek}, Mar 1 weekday ${new Date(y, 3, 1).dayOfWeek}"
+      );
+    }
+
+```log
+1700: Jan 1 weekday 5, Mar 1 weekday 1
+1800: Jan 1 weekday 3, Mar 1 weekday 6
+1900: Jan 1 weekday 1, Mar 1 weekday 4
+2000: Jan 1 weekday 6, Mar 1 weekday 3
+2100: Jan 1 weekday 5, Mar 1 weekday 1
+2400: Jan 1 weekday 6, Mar 1 weekday 3
+```
+
 TODO: test negative year support once we've got a
 story for Python whose datetime.MIN_YEAR == 1.