Small fixes to minmea_gettime

* Handle four digit years provided by ZDA in minmea_gettime
* Assume GPS epoch to allow dates prior 2000 to be parsed

minmea.c

@@ -619,7 +619,11 @@ int minmea_gettime(struct timespec *ts, const struct minmea_date *date, const st
 
     struct tm tm;
     memset(&tm, 0, sizeof(tm));
-    tm.tm_year = 2000 + date->year - 1900;
+    tm.tm_year = date->year % 100;  // ZDA parser stores 4 digit year, filter centuries here so raw information is still preserved for user.
+    if (tm.tm_year < 80)    // GPS epoch begins 1980, assume a year range of 1980 to 2079 for the library.
+    {
+        tm.tm_year += 2000 - 1900;
+    }
     tm.tm_mon = date->month - 1;
     tm.tm_mday = date->day;
     tm.tm_hour = time_->hours;