The password field must be filled. The encryped password consists of 13 characters from the 64 character alphabet a thru z, A thru Z, 0 thru 9, . and /. Refer to crypt(3) for details on how this string is interpreted.
The date of the last password change is given as the number of days since Jan 1, 1970. The password may not be changed again until the proper number of days have passed, and must be changed after the maximum number of days. If the minimum number of days required is greater than the maximum number of day allowed, this password may not be changed by the user.
This information supercedes any password or password age information present in /etc/passwd.
This file must not be readable by regular users if password security is to be maintained.