Evaluate necessity of corrected max working time check in timesheet

[gstuer]

Evaluate necessity of corrected max working time check ensuring timesheet's integrity
Is this check really needed to ensure that the timesheet's instances are correct or is it adequate to check this in the Checker instances. What problems could appear? How can we solve these problems?

Originally posted by @gstuer in #79

[larsk21]

This is linked to the use of the TimeSpan class. I would propose to add a new data type for clock time, e.g. 07:15 or 23:45, and use the TimeSpan class for positive and negative "time vectors".