You can set your payroll cut-off date under Settings > Payroll Settings > Cycles. You can select the date when your payroll cycle ends and determine the payroll month for your cycle.
If your cut-off date is the 25th of the month, you will normally want your payroll month to be the month in which your cycle ends: e.g. 26 Jan to 25 Feb cycle should be Feb payroll and not Jan payroll. Meanwhile if your cut-off date is on the 4th of the month, you will normally want your payroll month to be the month in which your cycle starts: e.g. 5 Jan to 4 Feb should be Jan payroll and not Feb payroll.
If your settings are as shown in the screenshot above, your cycle will be from the 25th of the previous month to the 24th of the payroll month.
If a new employee joins your company on 10 January, their salary will be paid for 10 to 24 January in January payroll. In February payroll, they will receive their full monthly salary for 25 January to 24 February. If they leave the company on 18 Mar, their salary will be prorated for 25 Feb to 18 Mar.
If you want to set different cut-off dates for different employees, you have the ability to create multiple payroll cycles to assign different employees to. Just click on +Add to create a new payroll cycle.