Feature Request: Basic math in Clock format

[null-von-sushi]

I would like to request the ability to add to/ subtract from a numerical value.
The reason for this is that I would like to display a non-Gregorian Calendar in my menu bar. This means that the year would not be 2025.
There's a few countries (Japan and Taiwan) that use 'Eras' for example.
An easy and somewhat flexible way to have the Era year displayed would be to just do something like {yyyy-1234} (substituting yyyy for whatever number).