Nevermind. Solved it already. Here is what I did for the rest of you.
1. I set a rule on the ''Start Button'' named ''Click''
2. I then created an action on the ''Click'' rule to set the vaule of the Textbox field to ''now()''
2. I then created a second action on the ''Click'' rulet o set the value of the Textbox to ''substring-after(txtStart, ...