Time Zone / Location error

I replaced my router & modem last week.  Ever since, my computer regularly tells me my time zone has changed, from Pacific to Eastern and back again.  No big deal on my home PC- I can turn off the feature to autodetect the time zone.  But on my work PC, this feature is administrator controlled and I can't turn it off, so I am pestered with continuous notifications of time zone changes.  How do I fix this?  The xfinity assistant just sent me in circles and said it didn't understand the issue.  Do I need to replace my new cable modem?