Edit: Oops, I read that wrong. The previous answer from Jan is correct one.
No need for custom macros. All you need to do is:
{% GetDateTime(CurrentDateTime, "MM/dd/yyyy")%}
or in your case:
{% GetDateTime(CurrentUser.UserActivationDate, "MM/dd/yyyy")|(identity)GlobalAdministrator%}
The macro takes standard C# formatting string from msdn
Richard