If you create accounts manually, you need to know about the makeup of usernames and passwords so you don't get slowed down by having them rejected by the system.
Usernames must be unique and are not case sensitive when you or your account holders enter them. They are always displayed in lower case.
If you are allowing email addresses to be used as usernames, then the email addresses will also need to be unique.
Password length must be between 8 and 20 characters and contain a mix of letters and characters that are not letters.
The safest way to reset user passwords is to trigger an activation email and let the account holder do it themselves.
About expiry dates
The account becomes expired ON the expiry date - i.e. it does not allow the user to log in on the expiry date. The same is true of any other types of expiry date that may be in use by your organisation's schema such as eligibility expiry or permission set expiry.
Expiry reminder emails, if enabled, are sent 30 and 15 days before expiry.