Without the external cue, the difference accumulates and so the internally regulated activities of the biological day drift continuously, like the tides, in relation to the solar day.

Moreover, there is a simple mathematical formula for measuring the cumulative annual drift of the day of the week from the start of the century.

Last year Michael k. Walters of the Air Force Institute of Technology and I came up with a different way of calculating the cumulative drift of the day of the week.