Ah, I see, you are trying to create a datefrom a combined string in your formaula, but its already a string by adding text to a date, so you'd end up with an invalid date.
Could you try this instead:
Launch("https://teams.microsoft.com/l/meeting/new?subject=MeetingPlanner&startTime=" &
Text(dte_Selectdate.SelectedDate, "yyyy-mm-dd") & "T" & Text(TimeValue(gal_UTCTimeZone.Selected.UTCtime), "hh:mm:ss") & "&endTime=" &
Text(dte_Selectdate.SelectedDate, "yyyy-mm-dd") & "T" &
Text(TimeValue(gal_UTCTimeZone.Selected.UTCtime + Time(0,30,0)), "hh:mm:ss") &
"&showQuickStartDisclaimer=false&isByoeDisabled=false&providerType=0&navCtx=deep-link",
{},
LaunchTarget.New)