to calculate, the calendar
厤 centers on systematic reckoning: calculating and organizing time into a calendar. The two given meanings are closely linked, as a calendar is a calculated system of days.
厤 combines 厂 (cliff) with 秝, which likely represents an even arrangement of grain stalks, suggesting orderly calculation. The exact historical development is uncertain, but the character is associated with reckoning and the calendar.
Under a cliff 厂, two grain stalks 禾 are arranged evenly 秝, like a farmer calculating the harvest and marking days on a calendar.
For レキ, imagine a reckoning clerk named Reki who calculates the calendar under the cliff: Reki -> レキ.