-replaced timegm implementation on windows. Fixes bug with MMS file times

src/common/conversions.c

@@ -50,11 +50,43 @@ time_t timegm (struct tm *tm)
 
 #ifdef _WIN32
 
+#ifndef _MSC_VER
+/* Algorithm: http://howardhinnant.github.io/date_algorithms.html */
+static int 
+days_from_civil(int y, int m, int d)
+{
+	y -= m <= 2;
+	int era = y / 400;
+	int yoe = y - era * 400;                                   /* [0, 399] */
+	int doy = (153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1;  /* [0, 365] */
+	int doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;           /* [0, 146096] */
+	return era * 146097 + doe - 719468;
+}
+
+/* from https://stackoverflow.com/questions/16647819/timegm-cross-platform */
 time_t 
-timegm(struct tm* tm_time)
+timegm(tm const* t) /* does not modify broken-down time */
 {
-    return mktime(tm_time) - _timezone;
+	int year = t->tm_year + 1900;
+	int month = t->tm_mon;          /* 0-11 */
+	if (month > 11)
+	{
+		year += month / 12;
+		month %= 12;
+	}
+	else if (month < 0)
+	{
+		int years_diff = (11 - month) / 12;
+		year -= years_diff;
+		month += 12 * years_diff;
+	}
+	int days_since_1970 = days_from_civil(year, month + 1, t->tm_mday);
+
+	return 60 * (60 * (24L * days_since_1970 + t->tm_hour) + t->tm_min) + t->tm_sec;
 }
+#else
+#define timegm _mkgmtime
+#endif
 
 #if defined(__MINGW32__)
 
@@ -169,7 +201,7 @@ getSecondsOffset(const char* offsetString)
 uint64_t
 Conversions_generalizedTimeToMsTime(const char* gtString)
 {
-    int gtStringLen = strlen(gtString);
+    int gtStringLen = (int) strlen(gtString);
 
     if (gtStringLen < 14) return -1;