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The community's VoIP FAQ contains this post here:
Jan 28, 2013 Question: Time and Date flashing or unable to set time correctly
Resolution: Please check => here <=
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Polycom Global Services
Often when I encounter this, the fix is to disable the NTP result from DHCP, as it has a tendency to override your manual settings.
Please ensure tcpIpApp.sntp.gmtOffset.overrideDHCP="1" is set, and that your other time-related values are proper.
If that doesn't work, we'll need more info to help you further.