After unification, problems with trying to work with two calendars (both depending upon constant observation) led to a merged, simplified civil calendar with twelve 30-day months, three seasons of four months each, plus an extra five days, giving a 365-year day but with no way of accounting for the extra quarter day each year. Day and night were split into 24 units, each personified by a deity. A sundial found on Seti I's cenotaph with instructions for its use shows us that the daylight hours were at one time split into 10 units, with 12 hours for the night and an hour for the morning and evening twilights. However, by Seti I's time day and night were normally divided into 12 hours each, the length of which would vary according to the time of year.