Islamic calendar
From Tardis Wiki, the free Doctor Who reference
The Islamic calendar was a human dating system to which the Sixth Doctor sarcastically referred when Peri Brown asked for the date in "human terms". According to him, the Gregorian equivalent to the year 2594 in the Islamic calendar was 3174. (PROSE: Burning Heart)
The TARDIS kept track of the Islamic calendar. (AUDIO: The Mouthless Dead)
