By default, the Timesheets module works on weekly basis, showing as a weekly period when you view timesheets.
This can be changed in the Timesheets General Settings where there are four options for timesheet periods:
To change your 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.
Select a day or date for the timesheet period to begin.
Scroll down, then click Save.
Next time you view timesheets, it will now show in your chosen period.