Cobot invoices coworkers once a month. By default this happens on the day of the month the coworker signed up, i.e. if someone signed up on March 23rd Cobot will generate an invoice every month on the 23rd.
To change that date, go to the coworker's page and click on "Change Invoice Date" in the "Invoicing" box.
Cobot will not calculate any prorated payments when you move the invoice date, it will just assume the set date to be the next invoice date.