Oxford Dictionary
timepiece,
noun,
an instrument, such as a clock or watch, for measuring time
Since my phone displays hours, minutes and seconds, its legal.
If I do 4 sectors in a day with 5mins worth of taxiing at each end + 10mins for a RunUp at the beginning of the day thats 0.6 added to the Switch for my Logbook.
If I had instead added a 'standard' 0.2 per sector that makes 0.8 to be added for my logbook.
That's a difference of 0.2 per day, 1.0 per week...... adding up isn't it. A years worth of logging like this and I could easily inflate by 40-50hours per year.
You need to be accurate with this stuff, avoid using the 'rule of thumb'. How you time it is up to you but I also use 'Blocks to Blocks' on a stopwatch. That "Start, Up, Down, Stop" idea also sounds pretty good though.