Menu bar clock differs from system time
I know you asked about a fix without restarting, but I really think your best option is to reset the NVRAM which, of course, requires you to restart your Mac. The reason for this is that your Mac's Non-Volatile RAM stores its timezone information.

Reset your NVRAM

To reset the NVRAM you'll need to follow these steps:


Fully shut down your Mac 
Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down for about 20 seconds (or until you know for sure your Mac has rebooted a second time)
Let go of the keys and let your Mac reboot normally.