Bug#26536 func_time failure on vm-win2003-64-b, occurs every time

- Avoid overflow in sec_since_epoch by shifting the time
   back 2 days for times close to the maximum range of my_time_t
 - Improve comment about why we need my_time_t
 - Patch will also fix timezone2.test

include/my_time.h

@@ -30,7 +30,13 @@ extern uchar days_in_month[];
 
 /*
   Portable time_t replacement.
-  Should be signed and hold seconds for 1902-2038 range.
+  Should be signed and hold seconds for 1902 -- 2038-01-19 range
+  i.e at least a 32bit variable
+
+  Using the system built in time_t is not an option as
+  we rely on the above requirements in the time functions
+
+  For example QNX has an unsigned time_t type
 */
 typedef long my_time_t;