From Wikipedia article The Cypher of the Monks:
The Cistercian numeral system, a numeral system that was used by the Cistercian order of monks in the 13th to 15th centuries of the Middle Ages, and has been used sporadically since then. It allowed writing numbers from 1 to 9999 as single compound characters.
The Cistercian numbers can be represented in a 5x7 dot matrix display, thus providing 4 digits to represent the time on a digital clock.
The Day and month can be represented using a distinguishable dot pattern