All components except years may be expressed as a single or double letter. A single letter means the value will be shown as a single digit if less than 10, or two digits if 10 or greater. A double letter means that the value will always be shown as two digits, with a leading zero as necessary. Years must always be expressed as four letters:
second
s or ss
minute
m or mm
hour
h or hh
day
d or dd
month
m or mm
year
yyyy
A single letter means the value will be shown as a single digit if less than 10, or two digits if 10 or greater. A double letter means that the value will always be shown as two digits, with a leading zero as necessary.
Months and minutes are both expressed as m and are distinguished by context.