+ (dating system)
From Tardis Wiki, the free Doctor Who reference
A dating system was in use on Kandoka's moon that featured a "+" symbol preceding a number.
A year had at least thirteen months. The eleventh month contained at least twenty-eight days, the twelfth at least nine, and the thirteenth at least thirty-five.
The Thirteenth Doctor and her companions visited Kerb!am in the year +135 after receiving a request for help. (TV: Kerblam! [+]Loading...["Kerblam! (TV story)"])