add timegm_with_gmtoff()

src/timestamp.c

@@ -9,6 +9,17 @@
 #include <string.h>
 #include <assert.h>
 
+
+DYNAMIC_API time_t timegm_with_gmtoff(timestamp* ts)
+{
+    const time_t raw_time = timegm(ts);
+    if(raw_time==-1)
+        return -1;
+    
+    return raw_time - ts->tm_gmtoff;
+}
+
+
 DYNAMIC_API timestamp * new_timestamp(time_t clock)
 {
     timestamp *ts = calloc(1, sizeof(timestamp));

src/timestamp.h

@@ -30,6 +30,17 @@ typedef struct _timestamp {
 typedef struct tm timestamp;
 #endif
 
+
+// timegm_with_gmtoff() - convert the broken-out time into time_t, and respect tm_gmtoff
+//
+//  parameters:
+//      timeptr(inout)     broken-out time; members will be "normalized" by this function.
+//
+//  return value:
+//      time_t that holds the usual "seconds since epoch"
+DYNAMIC_API time_t timegm_with_gmtoff(timestamp* ts);
+
+
 // new_timestamp() - allocate a new timestamp
 //
 //  parameters: