Thanks Ng... I should've known!
I thought that those settings would've resulted in the observed issue irrespective of the method with which the calendar entry was created.
What got me was the fact that it would work fine one way but not the other.
I removed the following parameters from my NOTES.INI file...
Timezone=-10
DSTLAW=10,1,1,4,1,1
DST=1
... and the issue disappeared.
Funnily enough, however, all of those settings re-appeared in my NOTES.INI after re-starting the Notes client.
There are no policy settings that establish those parameters, so I'm a little confused.
Even though the settings have re-appeared, the issue no longer presents itself... strange?!?!
Cheers!