If each record represents a day or part of a day worked, either a relationship or performing a find that uses a date range as criteria can be used to produce a group of records that can be totaled with a summary field.
Say your table of work records is called HoursWorked and you have another table called MonthlyCheck.
Define a date field, DateStart, format it with a pop up calendar and give it this auto-enter calculation:
Self - Day (Self) + 15
This will take any date selected from the pop up and produce a date for the 15th of the same month.
Add cDateEnd as a calculation field with date specified as the return type:
Date ( Month ( DateStart ) + 1 ; 14 ; Year ( DateStart ) )
This produces a date range from the 15th of one month to the 14th of the following month.
Define this relationship:
MonthlyCheck::DateStart > HoursWorked::Date AND
MonthlyCheck::cDateEnd < HoursWorked::Date
then define a calculation field, cWages as the HourlyRate * HoursWorked
Define a summary field as the total of cWages and place this field on your MonthlyCheck layout.
There are other approaches that will also work.