# WakeAtTime Schedules a callback from the timer. ``` virtual kern_return_t WakeAtTime(uint64_t options, uint64_t deadline, uint64_t leeway); ``` ## Parameters `options` The timebase to use when interpreting the `deadline` and `leeway` parameters. For a list of possible values, see [Clock Types](/documentation/DriverKit/3242783-clock_types). `deadline` The time at which to execute your action. The meaning of this parameter depends on the timebase you specified in the `options` parameter. `leeway` The maximum amount of time beyond the scheduled `deadline` that the system may wait before executing your action. Leeway values improve the system’s power usage by letting the system schedule timers at a more advantageous time. The system guarantees the execution of the timer’s action before the combined `deadline` and `leeway` values expire. ## Return Value [`kIOReturnSuccess`](/documentation/DriverKit/kIOReturnSuccess) on success, or another value if an error occurs. See [Error Codes](/documentation/DriverKit/error-codes). ## Discussion Use this method to schedule the execution time for your timer. Call this method from a block running on the same dispatch queue you passed to the [`Create`](/documentation/DriverKit/IOTimerDispatchSource/Create) method. If a previously scheduled timer has not yet fired, calling this method replaces the old time with the new value. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)