If changing the clock data of the PLC CPU does not change the clock data of the GOT, check that time adjustment is set on the GOT or in GT Designer3.
• GT37, GT27, and GT25
Select [GOT basic set] → [Time] → [Adjust] from the [Utility] menu.
• GT21
Select [Time Setting] → [Adjust] from the [Utility] menu.
• GT Designer3
◆ For GT Designer3 (GOT2000) Version 1
 Select [Common] → [GOT Setup] → [Basic Setting] → [Time Setting]. Select [Update the time setting] and [Adjust the clock], then set [Reference CH No.] and [Trigger Type].

If time adjustment is set, the GOT aligns with the clock data of the PLC CPU when powered on. Afterward, at the timing set for the trigger type, the GOT verifies its internal timer against the clock data of the PLC CPU and adjusts its clock to match the clock data of the PLC CPU. 
When [Trigger Type] is set to [Sampling], the GOT verifies its clock data at the specified interval (1 minute to 1440 minutes).