I've found that an hidden now() function that is relevant of a select_one button is responding with the start time of the form instead of the "when button is hit" time.
If I use exactly the same functions, but replacing the hidden type by a time type than it works properly. I've got the right "when button is hit" time (but without the seconds, that's why I prefer the hidden type...!)
Any ideas? Maybe hidden field-type can not be relevant of another field?