Same problem for me.
Reply from the support at HTC.
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This is not a fault with the device or its software, your device updates its time automatically from the internet and from your mobile provider.
Occasionally there is a difference in the time from the internet and the time from your provider which can cause the time to appear to be changing when it should be the same.
Normally this will resolve itself within a few days, however, this is very inconvenient so we recommend you disable any automatic time updates on your device.
WM 5, 6, 6.1, start settings, personal, phone, timezones and disable automatic time updates
WM6.5 old Sense: start, settings, all settings, persoanl, phone, UMTS services, automatic time updates
W>6.5 new sense, Settings -> Data services -> Time synchronization -> Untick all here
Sincerely,
Alex
HTC