A Baktun is the longest division on the Ancient calendar.
1 baktun is made up of 16 smaller divisions called katuns. Every katun is made up of 32 even smaller divisions called tuns, which are made up of 64 smaller winal cycles, which are made up of 128 kins. The end of the 128th kin moves the calendar ahead 1 winal cycle, after the end of the 64th winal cycle, the calendar moves ahead 1 tun. At the end of the 32nd tun, the calendar moves ahead 1 katun. On the 128th kin of the 64th winal cycle of the 32nd tun of the 16th katun, the transfer from the 128th kin to the 1st kin sends the winal, tun & katun scales back to 1 while the baktun scale moves forward to 2 (if the previous baktun was the 1st).