[rt-users] Scrip to add transaction-based CF 'Time Spent' into Ticket's 'Time Worked'

[Robert Matney]

Obligatory advance n00b warning. I have searched wiki, book and mail  
archives w/o success.

Goals:
1. Create transaction applicable Custom Field allowing users to track  
time (in hh:mm format) per transaction
2. Create a scrip which adds the value of this custom field to the  
current value of the 'Time Worked' field

Problem:
-> I do not see how to set validate the CF value as hh:mm
-> I am uncertain how to code the scrip

Can anyone point me in the right direction?
Thank you in advance.

Thank you,
Robert Matney