Calcluted Formula to get the last digit and the 2nd to the last digit of a number in a Column

So, 3 is for the month of March

For the 2nd to the last digit number.

Numbers 1, 2, and 3 a student should be enrolled within the working days from the 1st to 7th of the month;

4, 5, and 6 of the 2nd to the last digit Will be on the 8th to the 14th of the month.

7 and 8 on the 15th to the 21st of the month
then
9 and 0 on the 22nd to the last day of the month.

Based in the scenario above.
Can you help me please formulate a calculated formula to a column as an end date of enrollment in a year?

Example would be.
Student Number : 2019-6703
We get 3 (March) and 0 (22nd to the last day of the month.)

So, the student can only enroll on March 22 up to 31 the following year. Beyond that it is a penalty anymore.

I appreciate your help