Using modern sign numeration (such as found in the charts erected at astro.com, and by almost all contemporary astrological software), the decans are:
0 through 9degrees 59minutes 59econds = 1st decan
10 through 19degrees 59 minutes 59 seconds = 2nd decan
20 through 29degrees 59 minutes 59 seconds = 3rd decan
(Contemporary sign numeration goes from 0 to 29degrees 59 minutes 59 seconds of each sign; oldtime sign numeration-still in use as late as the mid-20th century in parts of Europe-went from 1 degree to 30 degrees)