how do I turn off workstation clock synchronization

We recently upgraded our office workstations from 98 to
XP. With the upgrade came automatic clock synchronization
from the network domain time server. We have an app that
uses the workstation clock to record time and date
activity in a database. We need to back date the
workstation to record the date gift certificates were
redeemed. How do we turn off the synchronization on the
workstation we are using while we enter the redeemed
certificates without disrupting the time server for the
entire network.