Can someone ELI5 why Unix timestamp would be stored as left-padded 48 bits for a 32-bit or potentially a 64-bit number? Is it a coincidence that 48 is the middl

Can someone ELI5 why Unix timestamp would be stored as left-padded 48 bits for a 32-bit or potentially a 64-bit number? Is it a coincidence that 48 is the middle of 32-64?