Accept and withdraw shifts
Accepting a shift
Open a released shift and tap Accept shift — if someone beat you to it, you'll see 'This one went to someone else' with any other open shifts, never an error page.
Withdrawing from a shift you've accepted
Once you've accepted, the Withdraw button on that shift's detail screen shows the exact cutoff time, in the shift's own timezone.
Past that cutoff, the button is replaced with a message telling you the window has closed and to contact your employer directly.