STD is a reference to the motor oil brand, STP. Its name also shares the initialism with 'sexually transmitted disease', keeping in line with Rockstar's crude humor.
There are at least two calendars in the Doherty Garage; one in the main service area on the office exterior wall and the other inside the office.
The calendar is open on the month of January and uses the Sunday-Saturday week format. The first day of the month is shown to be Thursday, which was the first day of 1987 so the garage must have been abandoned for at least five years.
Seen in the Zeroing In cutscene, in the service area.