feat: added duration to task to calculate end date

[lettuceDestroyer]

Added the feature to calculate the end date of a task based on the task duration. This is very similar to this pull request with the difference that it supports years, months, days, hours, minutes, seconds and milliseconds aswell as any combination of them.

[safwansamsudeen]

Thanks very much @lettuceDestroyer - I've adapted your code and pushed it. Apologies for not merging this sooner.