time period specification should not include the upper bound

Currently, if we specify a time period "1,12:00-12:01", it includes all seconds between 12:00:00 and 12:01:00, both ends inclusive.

That means, for instance, that if we wish to have an item checked exactly once a week, we have to specify the flexible interval above with a delay of 61 seconds (not 60!), which is somewhat non-intuitive.

It is proposed that the time period "1,12:00-12:01" includes seconds between 12:00:00 and 12:00:59.