Currently, when task finishes running, its next activation is calculated in relation to the time when task ended, not when it started.
This results in rare, yet problematic scenarios like:

• schedule repair to run once a week
• repair starts and runs for a week + 1h
• next repair activation takes place in a week - 1h, meaning that we skip one week's repair

This scenario can be found here: Ref: https://github.com/scylladb/scylla-enterprise/issues/5281

Here are the places in the code from which we can see next task activation calculation:

In order to fix that, SM should calculate the next task activation in relation to the task start time.