Two kinds of time are used in Almanacs; clock or mean time in some, and apparent or sun time in others. Clock time is always right, while sun time varies every day. People generally suppose it is twelve o'clock when the sun is due south, or at a properly made noon-mark. But this is a mistake. The sun is seldom on the meridian at twelve o'clock; indeed this is the case only on four days of the year: namely, April 15, June 15, September 1, and December 24. The time when the sun is on the meridian or at the noon-mark is also given to the nearest second, for every day in the year. This affords a ready means of obtaining correct time and for setting a clock by using a noon-mark, adding or subtracting as the sun is slow or fast.

Old-fashioned Almanacs, which use apparent time, give the rising and setting of the sun's centre, and make no allowance for the effect of refraction of the sun's rays by the atmosphere. The more modern and improved Almanacs, which use clock time, give the rising and setting of the sun's upper limb, and duly allow for refraction.