The arena time is displayed on a large wall clock in each tower, and each aircraft cockpit also has a clock.
To change the time in an arena for offline play or for hosting a LAN session, click Options->Arena Setup->Environment->Adjust Time on the clipboard menu.
The Adjust Time dialog shows the range of time for the arena clock and displays the current arena time. Be sure to enter a new time in HH:MM format.
You may try different values between 0 and 100 for the multiplier, then click Apply to view the effects in the arena.