Edit the record at any time: the change applies from the next calculation, and the audit trail keeps the previous and new values, who changed them and when.
Editing the record
Open the employee record and edit. Personal data, position, department, contract type and salary can be changed at any time; the change applies on the next "Calculate" of a period not yet approved. Approved or paid periods do not change.
Salary adjustments
Change the base salary on the record before calculating the period the raise applies to. If the month's period was already calculated but not approved, recalculate after saving. For a raise that only applies from the 15th, enter the difference as a premium that month and adjust the base salary for the following months.
What gets recorded
Every change enters the audit trail with the previous and new values, who changed them and when; that is what answers "when did their salary change and who changed it". Past payslips keep position, department and salary as they were at calculation time, so history is never rewritten.