Alternian calendar

Solar Sweep
Solar Sweeps are used as a measurement of time on Alternia. They are probably a Troll term for "years". Each solar sweep lasts approx. 2.17 earth years, meaning six Solar Sweeps are equal to 13 earth years, putting the Trolls at roughly the same age as the Kids.

Wriggling Day
Wriggling Days are the Troll equivalents of Earth birthdays, occurring once for each Troll each Alternian Solar Sweep. It just so happens that at the beginning of Act 5 we see our first Wriggling Day, that of Karkat Vantas, on "the 12th bilunar perigee of the 6th dark season's equinox", making him 6 Solar Sweeps (13 Earth years) old. The other Trolls are presumably all the same age.