Decipher the logic and find the number for September October November & December

Question

Here is a list showing the month and a number for each month. Now decipher the logic and find the number for September October November & December

January 100
February 233
March. 334
April. 460
May. 512
June. 645
July. 760
August. 823

Answer 1

september 9 5 6
october 10 0 1
november 11 3 4
december 12 5 6

Assume sunday - 0 , monday -1 and so on till saturday - 6

first digits are the month represented in numbers
Now last 2 digits
second digit is a normal year and third digits are leap year.

January 1 0 0
Normal year Sunday - 0 , Leap year - 0
Add 31 days

Feb 2 3 3
Normal year 3 wednesday , Leap 3 wednesday.
Add 28 for normal and 29 for leap

March 3 3 4
Normal year 3 wednesday , Leap 4 thursday
Add 31 days

April 4 6 0
Normal year 6 saturday , leap year 0 sunday

And continue so on. ...

If you don't get it take any normal calender year whose jan month starts from sunday and also any other calender of leap year whose jan month starts on sunday you will get the logic.
This only works for 2012 and 2017.