F_ADDPERIOD

Entrypoint addperiod compatible with UTF-8

   Input      CSTRING(254)     period 1 in pattern [d]:h:m:s
              CSTRING(254)     period 2 in pattern [d]:h:m:s
   Output     CSTRING(254)     sum of period 1 and 2

   Periods could be entered with 2- or 1-digit intervals (dd:hh:mm:ss or d:h:m:s).
   Negative periods starts with a "-" in front.
   Periods with pattern days:hours:seconds (4 intervalls) or hours:minutes:seconds (3 intervalls) are allowed.
   The output of periods with less than 3 intervals or containing other characters is <null> or an empty string.
   For input also f.e. '0:26:0:0' instead of '1:2:0:0' allowed for 26 hours.
   The output is always in pattern days:hours:minutes:seconds (4 * 2 digit intervals)
   TestSQL
   SELECT '00:01:25:10' AS ISCORRECT, F_ADDPERIOD('0:1:10:0', '0:0:15:10') FROM RDB$DATABASE;
   SELECT '00:01:25:10' AS ISCORRECT, F_ADDPERIOD('01:10:00', '00:15:10') FROM RDB$DATABASE;
   SELECT '00:00:54:50' AS ISCORRECT, F_SUBPERIOD('0:1:10:0', '0:0:15:10'), F_ADDPERIOD('0:1:10:0', '-0:0:15:10') FROM RDB$DATABASE;
   SELECT NULL AS ISCORRECT, F_ADDPERIOD(NULL, NULL) FROM RDB$DATABASE;