Button used for capturing time (Timer)

I am looking for help with the following idea I am trying to attach a worksheet to show this but here is the basic idea


When reading a water meter in a large apartment building
a meter reader can also click a timer (LAP) button as the meter
passes preset intervals. The worksheet would record the "lap times"
in separate cells which could then be compared and usage figures
can be calculated, allowing the meter reader to know if there is
currently a Leak or other problem before leaving the premesis.

In the attached worksheet you can see what I would like
the meter reader enters the reading - the light green "Interval List"
is created from that reading. The Meter reader then watches the meter
clicking the LAP button each time the interval is passed, the current time
is then stored to the Pink cells (Similar to the NOW() function), the times are then calculated in other cells like I did in the Blue column in this case showing elapsed time.

I have tried several ways to do this using NOW, but each time all the cells are updated to NOW not just the current cell - and the Single Button woulod be really nice so the meter reader didn't have to look away from the meter.