Tag visibility based on start and end dates

[BoPeng]

Is your feature request related to a problem? Please describe.
We use tags to track projects. My biggest fear of using a tag based system is the increasing number of tags over time. When we have hundreds of tags, it will become difficult to choose the right tags for time.

Describe the solution you'd like
Add start and end date (optional) dates for tags so that only active tags will be displayed.

Describe alternatives you've considered
Removing obsolete tags (completed tags) would not work since this will remove all related events.

[jmattheis]

I think something like "archiving tags" would be easier understand for users but yeah this sounds useful.

[BoPeng]

"archiving tags" would be easier understand

What to do if users want to edit time retrospectively? Having start and end dates has the advantage that tags can be active/inactive at different times. It can be even better to have "assigned_to" property for tags so that our users can only see the projects to which they are assigned.