The rules says wilderness encounters are start of the morning and evening watches, but obviously as the games mouster you’re welcome to ignore that and say eg encounters are nocturnal, they’re happening at night.
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Minor clarification: the game master rolls at the start of morning and evening watches to find out about wilderness encounters for that watch; however, I believe the rules state that if an encounter occurs, the GM should roll 1d12 to find what hour the encounter occurs in--the d12 result cooresponds to the spokes on this turn tracker, or the columns on the standard turn tracker. Wilderness encounters can happen any hour of the day. Edit to add: this was something I missed when I first read Mausritter.