diff --git a/include/boost/date_time/gregorian_calendar.ipp b/include/boost/date_time/gregorian_calendar.ipp
index f41c499..5a7e49b 100644
--- a/include/boost/date_time/gregorian_calendar.ipp
+++ b/include/boost/date_time/gregorian_calendar.ipp
@@ -77,7 +77,7 @@ namespace date_time {
     unsigned short a = static_cast<unsigned short>((14-ymd.month)/12);
     unsigned short y = static_cast<unsigned short>(ymd.year + 4800 - a);
     unsigned short m = static_cast<unsigned short>(ymd.month + 12*a - 3);
-    unsigned long  d = ymd.day + ((153*m + 2)/5) + 365*y + (y/4) - (y/100) + (y/400) - 32045;
+    unsigned long  d = static_cast<unsigned long>(ymd.day) + ((153*m + 2)/5) + 365*y + (y/4) - (y/100) + (y/400) - 32045;
     return static_cast<date_int_type>(d);
   }