Simply create an ICS file and import it into Google Calendar:
BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
RRULE:FREQ=MONTHLY;INTERVAL=1;BYSETPOS=-1;BYDAY=SU,MO,TU,WE,TH,FR,SA
SUMMARY:Timesheets
DTSTART:20240930T110000Z
DTEND:20240930T110000Z
SEQUENCE:0
DESCRIPTION:Timesheets and Invoices
END:VEVENT
END:VCALENDAR