The displayed measure
CanYouJust's deadline headroom is the time between calculated completion and the hard deadline, in calendar hours. It is not an inventory of unallocated productive staff hours.
If the project finishes Monday at noon with a Monday end-of-day deadline, it has about twelve calendar hours of headroom. That does not mean a part-time colleague can work another twelve productive hours that day.
Negative headroom is the amount by which the forecast runs beyond the commitment. A missing forecast or deadline cannot supply a meaningful headroom number.
Do not compare unlike units
History movement uses a preserved person's reference working calendar and is labelled working hours. Deadline headroom uses calendar time because an external deadline continues through breaks and nights. Both are useful; they answer different questions.
See targets and hard deadlines, capacity and baselines.