[Action Script] Architecture issue June,2012

Hi

I'm writing a parking meter application - it must calculate the payment based currentTime minus parkingBegin time.
The app must be fool-proof, also.
I've created 4 input fields - one for every digit in 24h format, input fields are limited to 0-2,0-9,0-6,0-9.
To access these input fields i've used ENTER_FRAME event, which calls function which checks whether input1 is filled, than advances to input2 and so on, thus making it possible to only enter proper digits in proper cells.

the question is following: after each calculation attempt triggered by user entering digit in input4 and pressing enter, the entire procedure must be started over. What is the correct way to return/close all open functions/procedures/calls/whatever.
I have no experience in OOP - this is my first app, so my question might sound a bit dumb...
Architecture issue

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