Fix task may appear to be triggered more than 1 second late

[progheal]

The calculation of next trigger time do not discard fractional seconds. Depending on the exact timing of ticking, the task may appear to be triggered more than 1 second late, despite properly ticked every second.

Example: Suppose a task is scheduled to trigger at 00 second, but the calculated next trigger time is at 00.5 second, while tick() is happening at every .35 second. At 00.35 second a tick happened, but because 00.35 < 00.5 the task is not triggered. It will be triggered in the next tick that's at 01.35 second, which appeared to be late for 1.35 second (despite get_delay() will return 1.35 - 0.5 = 0.85 second in this case).

The source of the fractional seconds is from the from argument, where now(), a clock that usually have sub-second precision, is passed. If during the calculation, date_changed is never true, the variable curr will only be modified through += and -= in the second half of the function, where the increments and/or decrements are all multiple of whole seconds.

This fix calculates the fractional second by modulo 1 second, then subtracted that from the calculated time. Since the trigger time is now scheduled at exact second, whenever tick happened within that second the task will be triggered. Doing floor by std::duration_cast<> is not good because it changes the duration type (because of different ratio), and therefore the the time_point type (because of different duration).

[PerMalmberg]

Hello @progheal

This fix seems correct, thank you for tracking it down and creating this PR.

At the moment I'm fully occupied with my summer vacation away from coding so I won't setup my environment to run the unit tests to verify it haven't broken anything. As such I will leave this PR open until that can be arranged.

What we really need is GH Actions to run these. Should you have the time to set that up I'll happily merge that too.

Edit: Also, please add a comment to the new code so it is clear why it is needed. I know it can be found via Git, but I think a comment is worthwhile here.

[progheal]

Comment added. Have a nice vacation!

About setting up GH Action, unfortunately I don't have much free time either. I've skimmed through the documents, but because I have never used them before, I don't really know how many time it will take. Sorry about that.

[PerMalmberg]

Tests passed :)

Thanks for your contribution.