By default, when a crew member joins your vessel on workrest, their start date will be the date they joined.
Within your Hours : Review, you'll see this date confirmed in the footer of their calendar and by a thick half circle on the day itself (marking their start date).
You will need to confirm their start date in order to complete their timesheet either by confirming the original date shown or by changing it to a different date.
To confirm the date shown:
Click the 'Confirm' button
To change the date:
Click within the date area
A calendar will pop up (shown below)
Select their start date from the popup calendar
Click the 'Confirm' button
You can further amend the start date of crew via their User page Settings : Manage Crew page by editing the date in the 'Start date' column.