The format string link is located on MSDN: user-defined date format specifications .
What you are looking for is:
yy Represents the year as a two-digit number. If a year has more than two digits, the result is only two low order digits. If there are less than two digits in a year, the number is filled with leading zeros to achieve two digits.
MM Represents a month as a number from 01 to 12. A one-month month is formatted with a zero value.
dd Represents the day of the month as a number from 01 to 31. A one-day day is formatted with a zero value.
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