Using date you specify the following format:

date -r test.txt +'%Y%m%d%H%M%S' 

1. %Y: Year (with four digits)
2. %m: Month number (with two digits)
3. %d: Day (with two digits)
4. %H: Hour (from 00 to 23)
5. %M: Minutes
6. %S: Seconds

You can check man date to more information.

Using date you specify the following format:

date -r test.txt +'%Y%m%d%H%M.%S' 

1. %Y: Year (with four digits)
2. %m: Month number (with two digits)
3. %d: Day (with two digits)
4. %H: Hour (from 00 to 23)
5. %M: Minutes
6. %S: Seconds

You can check man date to more information.