Time turner question

[ryanweasley]

No: HPFGUIDX 91711

I was just wondering... I understand how a time turner is supposed to 
work but, how were Hermione and Harry able to use theirs toward the 
end of POA to go back and rescue Sirius? (as well as themselves) 
There was no way that they could have escaped the Dementors to even 
have a future to travel back from. And yet they did? I'm sorry if 
this has been explained or if no one understands what I'm even asking 
but it is really bugging me. Someboby help!

"...No need to tell me your name. Red hair, and a hand-me-down 
robe... You must be a Weasley."    -RyanWeasley