According to Wikipedia, Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Windows AD needs timestamps for resolving AD replication conflicts and for Kerberos Authentication. Kerberos uses them to protect against replay attacks—where an authentication packet is intercepted on the network and then resent later to authenticate on the original sender's behalf.

When a Windows server receives a Kerberos authentication request, it compares the timestamp in the request to its local time. If the difference between the local time and the timestamp is too big, the authentication request is rejected and Kerberos authentication fails.

Properly synchronizing your network with a consistent and accurate time source is very important. However, it's not enough to simply synchronize the time on your network devices—this effort should extend all the way to the desktop.