There are 4 related time and date values that can be set in a record:
Knowing any 3 of them allows the 4th to be computed. That can be useful in assessing the practicality of the work that remains to be done. The estimates appear following the corresponding field display.
Weekends and holidays are not included in the estimates. That of course doesn't account for other staff absences, such as vacations.
When estimating for an open record, the time worked is taken into account, with the effective start date being now if the start date has passed. For a closed record, the actual start date and estimated time are used.