to calculate
祘 centers on the act of calculating or counting. It is a rare kanji whose meaning is straightforward but not commonly used in modern Japanese.
祘 is composed of two 示 elements side by side. The exact historical development is uncertain, but the doubling of 示 may have been used to emphasize the idea of counting or calculating, possibly related to ritual or display.
Two 示 (altar/display) elements side by side suggest laying out items for counting, like arranging offerings to calculate their number.
For サン, imagine using the sun (サン) to count the days: each sunrise helps you calculate time. サン -> サン.