Time stamps

Resurrecting the time stamp issue, as I'm trying to collect the time at multiple spots in a form (using XML forms).

I've tried creating a type "acknowledge" then using ${acknowledge} = 'OK' as a relevant condition for the following two strings calculating the hour and minute:
once(format-date-time(now(), '%H'))
once(format-date-time(now(), '%M'))

But this returns the hour and minute when the form was first opened; it doesn't calculate after the acknowledge trigger. Based on @Jorge_Durand_Zurdo post, what I'm trying to do seems possible, but I've run out of ideas on how to implement this.

Thanks in advance for any help! -- Greg