Tickets' time is drawn from the hours included each month - for that month only - and from any hours bought on the contract, before it is billed.
days
While the contract is in force, time bills at these rates - the after-hours one for tickets opened outside business hours - and new invoices are due on these terms. Leave one empty and the default in Admin > Settings > Defaults applies.
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From Admin > SLAs. While the contract is in force, the client's new tickets get them, and the default for a priority left on the default.