This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
The Long Count is really a mixed base-20/base-18 representation of a
number, representing the number of days since the start of the Mayan
era. It is thus akin to the Julian Day Number (see section 2.15).
The basic unit is the "kin" (day), which is the last component of the
Long Count. Going from right to left the remaining components are:
uinal (1 uinal = 20 kin = 20 days)
tun (1 tun = 18 uinal = 360 days = approx. 1 year)
katun (1 katun = 20 tun = 7,200 days = approx. 20 years)
baktun (1 baktun = 20 katun = 144,000 days = approx. 394 years)
The kin, tun, and katun are numbered from 0 to 19.
The uinal are numbered from 0 to 17.
The baktun are numbered from 1 to 13.
Although they are not part of the Long Count, the Mayas had names for
larger time spans. The following names are sometimes quoted, although
they are not ancient Maya terms:
1 pictun = 20 baktun = 2,880,000 days = approx. 7885 years
1 calabtun = 20 pictun = 57,600,000 days = approx. 158,000 years
1 kinchiltun = 20 calabtun = 1,152,000,000 days = approx. 3 million years
1 alautun = 20 kinchiltun = 23,040,000,000 days = approx. 63 million years
The alautun is probably the longest named period in any calendar.