Calculating monthly salaries with actions

If an action occurs on the first of the month or if no action occurs that affects salary, the full month's salary is the salary used for that month.

If an action that affects the salary does not occur on the first of the month, the full month's salary is the sum of partial salaries calculated for each applicable employee using this formula:

(Monthly Salary A * Calendar Days Salary A in Effect / Calendar Days in Month) + (Monthly Salary B * Calendar Days Salary B in Effect / Calendar Days in Month)