Janus

The god of Eternal Becoming, Janus is always moving between the end and the beginning. He oversees all beginnings, passages, and state changes. He's there for births and deaths, and at the beginning of new endeavors, and he places his hand on the first stone of constructions. The great King Numa Pompilius dedicated to Janus the month following the winter solstice, Januarius, because that is the moment in which the sun comes back to life and the cycle of the year begins anew.
Janus is portrayed with two faces, one that looks back to the past and one that looks forward to the future. For this reason, he's the protector of seers. He's portrayed as an old man, because he's the father of time, and he carries in his arms a key and a stick, like the humble porters. His main temple is in the Forum Romanum, and it's a very important symbol. It is opened when the Empire goes to war, and closed again in peacetime. He doesn't have a flamen specifically dedicated to his cult, because his priest is the Rex Sacrorum himself, the most important priest of Rome, who is first in processions and religious ceremonies, coming even before Jupiter's Flamen.