Managing password expiry for a user account (Unix/Linux)

I find this very useful on several occasions so here we go. Login as root and execute the following:
passwd -x -1

So if you want to remove password expiry from bob's account, type:
passwd -x -1 bob

Notice that -1 parameter after -x represents number of days before the password expires. Since we do not want the password to expire, we have set it to -1. However, you can set it to any value you want. So, if you want your password to expire after 90 days (i.e. 3 months) for bob's account, type:
passwd -x 90 bob