Visualize The Extended Month
Think of the Zodiac sign as starting at its start day (21) and extending to the end day (22) + number of days in loMo (30)
Any date that is less than or equal to the last day of loMo is in the loMo
Any date that is more than the last day of loMo is in hiMo, and is too large by the number of days in loMo
21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 ... 21 22
21 22 23 24 25 26 27 28 29 30 31 32 33 34 34 ... 51 52