Looking up to the cliff, you see the trees, and the sun shining down on them. Using the sun and the trees (up there on the cliff), you've devised a method to tell what time of the year it is, based on where the shadow from the trees fall. After a while, you develop a bigger system. This system is a whole calendar.
Remember the word 歴史, which means "history"? Using a calendar, you too can remember history (otherwise, who knows when these things happened). So, the reading for this kanji is the same as the reading for 歴史's beginning, because they're related.