Scrip to add transaction-based CF 'Time Spent' into Ticket's 'Time Worked'

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


  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

-> 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