diff --git a/icu4c/source/i18n/simpletz.cpp b/icu4c/source/i18n/simpletz.cpp
index 03b710b36be..ceb55e4f469 100644
--- a/icu4c/source/i18n/simpletz.cpp
+++ b/icu4c/source/i18n/simpletz.cpp
@@ -460,7 +460,9 @@ SimpleTimeZone::getOffset(uint8_t era, int32_t year, int32_t month, int32_t day,
         || millis < 0
         || millis >= U_MILLIS_PER_DAY
         || monthLength < 28
-        || monthLength > 31) {
+        || monthLength > 31
+        || prevMonthLength < 28
+        || prevMonthLength > 31) {
         status = U_ILLEGAL_ARGUMENT_ERROR;
         return -1;
     }