There are three calendars in use by the arriving fleets.
The Collective uses the New Era Calendar that begins with the Artifact’s first message to humanity. Accordingly, the date of Gabriel’s arrival is 501 NE, Day 117.
The Empire uses the Imperial Calendar, a count of standard days since the founding of the Empire. On this calendar, Gabriel arrives on Day 266,072 of the Empire.
The Theists use a custom calendar based on the start of their journey. Accordingly, Gabriel arrives on Saturday, Day 205 of Journey Year 462.
Also, see the Timeline.