Never had this issue. Its just as simple as start to work without contract and the promise of department head to get a contract and after two weeks mention to the contracting that you work since two weeks and have still not signed a NDA.
Next sentence is: I don't fear to not get my money, but currently I don't know if you pay or someone else...
compiler error: `since(time_point_t)` cannot be called with `time_duration_t`
were you trying to use `for(time_duration_t)` ?