Dylan:

That is a reasonable explanation, but it makes the counter utterly useless. How can I be sure that a small volume has traded a minute ago? Maybe it was a large volume, but there was also a large volume yesterday at the same time!

An alternative is to reset the counter to zero at every GMT, or to never reset the counter at all. Either way would allow you to take the difference in values between two times as the volume traded between those two times, which only seems logical to me, no?