SetTime_t: Use the sign-fixed modulo we computed

* In C++03 and earlier, the sign of the modulo result is implementation
defined (this was fixed in C++11).
* Setting a negative time is not what we want here, so make sure we use
the proper value.

src/kits/support/DateTime.cpp

@@ -1410,7 +1410,7 @@ BDateTime::SetTime_t(time_t seconds)
 	}
 
 	BTime time;
-	time.AddSeconds(seconds % kSecondsPerDay);
+	time.AddSeconds(timePart);
 	fTime.SetTime(time);
 
 	BDate date(1970, 1, 1);