1 files changed, 34 insertions, 0 deletions
diff --git a/src/datetime.c b/src/datetime.c
index 3db5f7f..9048822 100644
--- a/src/datetime.c
+++ b/src/datetime.c
@@ -51,3 +51,37 @@ void datetime_tai(struct datetime *dt,datetime_sec t)
   dt->mon = mon;
   dt->mday = day + 1;
 }
+
+/* roughly 100x faster than mktime() */
+
+datetime_sec datetime_untai(struct datetime *dt)
+{
+  int year;
+  int day;
+  int mon;
+ 
+  year = dt->year + 1900;
+ 
+  mon = dt->mon;
+  if (mon >= 2) { mon -= 2; }
+  else { mon += 10; --year; }
+ 
+  day = (dt->mday - 1) * 10 + 5 + 306 * mon;
+  day /= 10;
+ 
+  if (day == 365) { year -= 3; day = 1460; }
+  else { day += 365 * (year % 4); }
+  year /= 4;
+ 
+  day += 1461 * (year % 25);
+  year /= 25;
+ 
+  if (day == 36524) { year -= 3; day = 146096; }
+  else { day += 36524 * (year % 4); }
+  year /= 4;
+ 
+  day += 146097 * (year - 5);
+  day += 11017;
+ 
+  return ((day * 24 + dt->hour) * 60 + dt->min) * 60 + dt->sec;
+}