adding support for expressions of the for {x+/-y} to allow for offsets

[dbcSolid]

I use ElevenClock in my organization to display the current work week (%W) using the custom formatting options (it's very useful thanks!). Unfortunately my organization uses a different definition of work week so some years there is an off by one error. It looks like ElevenClock uses the strftime C function which defines week 1 as starting on the first Monday. There is an ISO standard (ISO 8601) that defines things differently (Week 1 is the first week with a majority of its days in Jan), and my org uses a third definition.

My recommendation is to allow offsets/expressions in the custom data formatting to solve this problem more generally. e.g

%a {%W+1}.%w to add 1 week

I imagine it could be useful for other offsets I'm not thinking of.

I'm open to other solutions, but I have already implemented a fix for myself using a simple evaluation function that supports basic addition and subtraction inside {}'s as above that wraps the calls to strftime.

If there is interest I would be happy to create a pull request so others could benefit. It's a relatively small change, but I personally know a bunch of folks that would find it very useful.

[dbcSolid]

I added this expression parser to the settings panel so you can see the preview as well as where I think it is needed in init.py so it shows up correctly in the taskbar (if I missed any other places it is needed, should be easy to add). Currently it only supports addition and subtraction for simple offsets (I can't fathom a need to multiply a date) but should be easily expandable to more complex expressions if desired

[marticliment]

I will release a beta soon

[marticliment]

Hello,

a beta release has been published containing your changes:
https://github.com/marticliment/ElevenClock/releases/tag/4.2.0-beta