I have a Summit 800 that gains about 10 seconds a day. Is there a way to correct this without actually changing the time every couple weeks?
Sorry if this is a repeat question. I find it very difficult to search on this forum without getting results from other phone system brands.

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Try looking at Automatic Time Synchronizationin the manual Page 54:

https://www.rjillc.com/resources/User_Guides/Summit-&-Summit-800-F&O-manual-Issue1_2.pdf

Have you set up "NTP Attributes" to automatically sync to a time server such as time-a.nist.gov?