A "gross" equals 144 or 12 x 12
The number makes no sense since twelve hundred is one thousand two hundred, and twelve thousand is one ten thousand and 2 thousands. You can't have 2 numbers occupying the same space in a number...
If you want all the numbers added, it's 13212. If not, then it is impossible to write or type twelve thousand twelve hundred twelve in simplest form.
12am is midnight. 12pm is noon.
24