Discharge date and days remaining from your enlistment
The enlistment day counts as day one, and the discharge date is the day before the service period completes. For an 18-month term, that is the day before the same date 18 months later.
Training content and the nature of the duties differ, and service lengths are set per branch by policy. Those lengths change with legislation, so check the current standard.
Ordinary leave counts. Some absences, such as unauthorised absence or certain medical leave, may not, and the discharge date shifts back accordingly. Individual cases are decided by your unit.