calendar, era
曆 centers on the system of tracking time: a calendar that organizes days, months, and years, and by extension the eras or periods marked by that system.
曆 combines 厤 (likely a phonetic component related to order or sequence) with 日 (sun, day), suggesting the ordering of days. The exact historical development of 厤 is uncertain.
The outer shape 厤 looks like a structured framework, and inside sits 日 (sun, day). Picture a framework of days organized into a calendar, marking the passing of eras.
For レキ, imagine a calendar marked with a lucky (ラッキー) day: lucky -> レキ, and you flip through the calendar to find it.