diff --git a/bacnet-stack/src/datetime.c b/bacnet-stack/src/datetime.c
index d50c9a16..099f3232 100644
--- a/bacnet-stack/src/datetime.c
+++ b/bacnet-stack/src/datetime.c
@@ -131,7 +131,7 @@ static uint32_t day_of_year(
     return (days);
 }
 
-void day_of_year_into_md(
+static void day_of_year_into_md(
     uint32_t days,
     uint16_t year,
     uint8_t * pMonth,
@@ -215,7 +215,7 @@ uint32_t datetime_days_since_epoch(
     return days;
 }
 
-void days_since_epoch_into_ymd(
+static void days_since_epoch_into_ymd(
     uint32_t days,
     uint16_t * pYear,
     uint8_t * pMonth,