hitting the start timer button is the core functionality - it marks a
segment on a particular date. if you stop the timer, then start it
again, you will see two segments in the same date, but different
"session" - don't know what else to call them. the point here, for me
at least, is that it allows me to see the total calories/miles/etc.
for a single date.

alternatively, if you reset the lap, then you get separate entries for
the same date. this means that if you want totals for that date, you
need a calculator (and and expensive watch).

if you just push the lap button once - don't hold it to reset it, it
will switch to a new lap, but same date. similar to the abovemost.