Merge char values

Question

i have one integer number which is unique userid for my table ,i have generate it dynamically

like following

first i get the currunt date ans concatinate it in to the string

DateTime _date = DateTime.Now;

        

int day = _date.Day;

        int month = _date.Month;

        int year = _date.Year;

        int i=0;

        string uid = "" + day + "" + month + "" + year + "" + ++i + "";  

but insted of ++i i want auto generated value based on previous inserted id,,,

so that i first fetch previous id from database table 

and i got like this 1111201304

red is date and green is auto generated number

so now how can i remove first 8 digit so that perform addition operation on remain digits.......