This medallion reminds me of ancestral worship. Tribes who practice ancestral worship usually decorate using things that remind them of where they come from. Some of these things are skulls in coins or actual skull decorations. These tribes believe that their ancestors watch over them so they pray to their ancestors and worship them as a link to the gods. This coin medallion is meant to be a replica of an Aztec gold coin which was made by an ancestor worshiping tribe. The skull and calendar could mean it was meant to symbolize an important time or date for the tribe.