add preference to ignore the date of the initial entry when computing daily duration mean statistics/graph
Seems that the daily duration mean can get a little confused by an initial sleep period that is prior to midnight. It thinks one should have slept the target number of hours on that date. That might not work out if a person goes to sleep at 10pm. In that case, it will only have 2 hours on the initial date that results in an invalid daily mean.
#35 ignored the initial entry for balance statistics. Perhaps something similar should happen for duration means.