Because sometimes a tenancy start may change whilst negotiations are ongoing, we don't start calculating rents straight away.
The start tenancy and start calculating rents button will appear when any of the following occurs:
if the fixed term from date is earlier than today
if the "first rent paid" tick box in the progress section has been ticked or,
if the "keys handed over" tick box has been ticked.
Once any of those events occur, a "start tenancy and start calculating rents button" will appear.
Clicking the button will save all the data in its current form, add a new rent due for the first payment of rent and add a scheduled rent for whenever the next rent is payable in the future. These will be shown in the rent schedule at the bottom of the tenancy view.
If the "first rent paid" tick box was ticked and amount entered, the rent schedule will also contain that first payment.