\define newMeetingAction()
<$action-sendmessage
    $message="tm-new-tiddler"
    title="New Meeting"
    tags="Meeting"
    at=<<now [UTC]YYYY0MM0DD0hh0mm00000>>
    participants=""/>
\end

<$button actions=<<newMeetingAction>>>
  New Meeting
</$button>

Note the use of [UTC] in the format string for the now macro. If you don't tell TiddlyWiki to use UTC, the time will be filled into the at field in your local time zone. Since TiddlyWiki assumes that date fields contain UTC timestamps, the time will get “converted” again when you display it, yielding an incorrect time (off by whatever your UTC offset is).

You can hop ahead to Working with Dates if you'd like to learn more about this.