How can I get all the computers on my LAN (which isn't connected to
the Internet) to have the same time.
Designate one machine as a the NTP (Network Time Protocol) server.
On that machine modify the registry as follows:
Value Name: LocalNTP
Data Type: REG_DWORD (DWORD Value)
Value Data: (0 = disabled, 1 = enabled)
On the playback machines point to the server via:
Control Panel > Date & Time > Internet Time > Enable