Calling-hours rules exist to protect the person receiving the call. Telerva therefore derives them from the recipient's number rather than from your account.
How the timezone is determined
The number itself carries the information: country code, and within some countries the area code. The platform resolves the jurisdiction from the number and applies that jurisdiction's rules.
This is more reliable than asking you to record a timezone per contact, which is a field that goes stale, gets imported blank, and is wrong exactly when it matters.
Jurisdictions
Rules differ by territory and the platform keeps them keyed by jurisdiction rather than applying one global window. A number that cannot be resolved to a jurisdiction is treated conservatively rather than optimistically.
What you see
A contact skipped for calling hours is reported as skipped with that reason, and becomes callable again when the window opens. It is not a failure and does not need retrying by hand.