Bo9 is correct. Time on the S8100 is administered through the Windows time settings.
Also, be aware the the 6400 series phones are notorious about time issues. They have an internal clock and only request a time update from the PBX when they are first plugged in, reset from the PBX, or when an administered time/date button on the phone is pressed. The internal clock tends to drift from the actual time on the PBX. You should administer "system-paramaters features" to "system updates time on phone display" to yes. The page on which that appears depends on your software release, but it should be around page 5 or 6. And for those users that still complain, administer a date/time button on their 6400 series phone. Pressing that button will force a time synce between the PBX and the 6400 display.
Statements expressed in this post are my own opinions and should not be considered as the opinion of my employer.