The section on TfL's website dealing with peak and off-peak times specifies the times but it doesn't specify if those are considered when card holders touch in or when they touch out.
Say, I board a train at 18:45 from Euston and arrive at Watford Junction at 19:07. The journey started during the peak time but ended during the off-peak time. What fare will I be charged - the peak or the off-peak?