When adding new starters you will have the option to enter their continuous service date, which can be different than the start date in their current post and determines their length of service.
If you want all users to have their leave allowance based on their continuous service date:
Go to Setup and click Settings.
Click System Settings, then Holidays and Leave Accrual.
Scroll down to Entitlement for Period to Use Continuous Service Date and Anniversary Leave Use Continuous Service Date.
Entitlement for Period to Use Continuous Service Date: If turned on, then the period entitlement will be calculated from the Continuous Service Date rather than Start Date. If turned off, it will always calculate period entitlement from Start Date.
Anniversary Leave Use Continuous Service Date: If turned on, then when calculating an employee's full entitlement based on years of service it will go back to Continuous Service Date if there is one rather than Start Date. If turned off it will always use Start Date.
Disable or enable the toggles as needed.