Effect of Daylight Saving on Recurring and Scheduled Tasks

For users in countries that adjust the clocks in the spring and autumn for daylight saving during the summer, the system ensures that all tasks run. The clocks change in the early hours and unless a task is due to run during the hour's change, the tasks continue to run at the set times.

Even if a task is due to run during the hour's change, the system has been designed to ensure each task is run.

Using the UK as an example, the clocks go forward on the last Sunday in March to what is called British Summer Time (BST) and go back on the last Sunday in October to Greenwich Mean Time (GMT).

Recurring Tasks

In the spring, the clocks go forward an hour at 1.00am, therefore the hour between 1.00am and 2.00am does not exist for that day. If a task is due to run at 1.30am, the system will run the task, for this day only, as if the hour has not changed and the task will show as being run at 2.30am BST. The following day and thereafter it adjusts the time and the task will run at 1.30am BST.

In the autumn, the clocks go back at 2.00am to 1.00am. Therefore, the hour between 1.00am and 2.00am occurs twice. For the task due to run at 1.30am, the system is programmed so that it runs the 1.30am task for the first occurrence of the 1.00am and 2.00am hour, i.e., BST, only. The following day and thereafter, it adjusts the time and the task will run at 1.30am GMT.

Scheduled Tasks

If a scheduled a task is set to run at 1.30am on the morning the clocks go forward, as the hour between 1.00am and 2.00am does not exist for that day, the system will run the task as if the hour has not changed and the task will show as being run at 2.30am BST.

If a scheduled task is set to run at 1.30am on the morning the clocks go back at 2.00am to 1.00am, as the hour between 1.00am and 2.00am occurs twice, the system runs the task for the first occurrence of the 1.00am and 2.00am hour, i.e., BST, only.