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I don't think we'll get far with [your] users being able to use the calendar
from the system tray. There's no notion of read-only access to the
Calendar. If you're in either the Administrators or PowerUsers groups, you
can open the applet and change the time, etc. If you're in neither, it does
not even open. Read-only access would be better, but the current behavior
is by design, so we shouldn't expect it to change short term.

The behavior is described in Q300022 "You Cannot Set the Date, Time, or
Time Zone on Your Computer"
http://support.microsoft.com/default.aspx?scid=kb;en-us;300022

An alternative would be to have someone create a quick app (or look for some
code examples) to implment a simple calendar pop-up.

[Microsoft]

P.S. the tie-in between authentication and time is part of the
Kerberos standard. Keeping the Server and client roughly in sync w.r.t. the
current time plays an important role in preventing replay attacks. In such
an attack, a 3rd party would sniff credentials on the network and try to
resend them to the server, pretending to be the original client. A common
understanding of the current time at both client and server assists in
detecting and preventing such an attack.

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