BDateTime: Time_T functions return or take a time_t

* They used an unsigned int, which led to overflows when trying to set
them to a time before January 1st, 1970 (local time)
 * Some things use January 1st, 1970, GMT (or UTC) as a reference point.
In my timezone this leads to such a negative date. An example is cookie
expiration dates which are set to this date to expire them immediately.
Spotted by Opera testsuite.
 * This makes the method unuseable for dates after 2036 (signed 32-bit
time_t will overflow then. This gives us just 33 years to switch to a
64-bit time_t. In te meantime, please try using other methods to set the
date and time for BDateTime objects if you need to go this far.

headers/os/support/DateTime.h

@@ -189,8 +189,8 @@ public:
 			const BTime&		Time() const;
 			void				SetTime(const BTime &time);
 
-			int32				Time_t() const;
+			time_t				Time_t() const;
-			void				SetTime_t(uint32 seconds);
+			void				SetTime_t(time_t seconds);
 
 			bool				operator!=(const BDateTime& dateTime) const;
 			bool				operator==(const BDateTime& dateTime) const;

src/kits/support/DateTime.cpp

@@ -1371,7 +1371,7 @@ BDateTime::SetTime(const BTime& time)
 	1.1.1970 - 00:00:00. If the current date is before 1.1.1970 the function
 	returns -1;
 */
-int32
+time_t
 BDateTime::Time_t() const
 {
 	BDate date(1970, 1, 1);
@@ -1392,7 +1392,7 @@ BDateTime::Time_t() const
 	tm_struct.tm_isdst = -1;
 
 	// return secs_since_jan1_1970 or -1 on error
-	return int32(mktime(&tm_struct));
+	return mktime(&tm_struct);
 }
 
 
@@ -1401,8 +1401,14 @@ BDateTime::Time_t() const
 	1.1.1970 - 00:00:00.
 */
 void
-BDateTime::SetTime_t(uint32 seconds)
+BDateTime::SetTime_t(time_t seconds)
 {
+	time_t timePart = seconds % kSecondsPerDay;
+	if (timePart < 0) {
+		timePart += kSecondsPerDay;
+		seconds -= kSecondsPerDay;
+	}
+
 	BTime time;
 	time.AddSeconds(seconds % kSecondsPerDay);
 	fTime.SetTime(time);