diff --git a/lib/timeutils/timeutils.c b/lib/timeutils/timeutils.c
index c897131c65..19d3ddbdd3 100644
--- a/lib/timeutils/timeutils.c
+++ b/lib/timeutils/timeutils.c
@@ -204,7 +204,7 @@ mp_uint_t timeutils_mktime(mp_uint_t year, mp_int_t month, mp_int_t mday,
         }
         mday += timeutils_days_in_month(year, month);
     }
-    while (mday > timeutils_days_in_month(year, month)) {
+    while ((mp_uint_t)mday > timeutils_days_in_month(year, month)) {
         mday -= timeutils_days_in_month(year, month);
         if (++month == 13) {
             month = 1;