By default, the Timesheets module works on weekly basis, showing as a weekly period. This can be changed in the Timesheets General Settings.
There are three options for timesheet periods:
Change timesheet period
Click on your name on the top right, then click Settings.
On the settings menu, click TIMESHEETS, then click General.
Scroll down to Timesheet period, then select your preferred period.
Scroll down, then click SAVE.
Next time you view timesheets, it will now show in your chosen period.