Hi @abroad36 :
I've made a test but did not encounted the problem you mentioned.
I suggest you to check according to the following steps:
Step1:Considering that the error message displays "The value of parameter ‘Start’ is invalid". I suggest you check the value of the variable StartDateTime.(File-Variables-Glabal)
1\Does the value of StartDateTime exist?
2\ Is the data type of StartDateTime ‘DateTime’

Step2:Check whether the formula is correct (ConfirmationScreen-OnVisible)
If(
IsBooking,
UpdateContext({ShowLoading: true});
Set(AvailableRoomsCounter, 1);
If(
!BookForMeeting,
Office365.V2CalendarPostItem(
MyCalendar,
User().FullName & "'s Booking",
StartDateTimeUTC,
EndDateTimeUTC,
{
RequiredAttendees: RoomsGallery.Selected.Email,
Location: RoomsGallery.Selected.Name,
Importance: "Normal",
ShowAs: "Busy"
}
),
ClearCollect(
RequiredAttendeesNoRoom,
Split(
MeetingsGallery.Selected.RequiredAttendees,
";"
)
);
ForAll(
AllRooms,
RemoveIf(
RequiredAttendeesNoRoom,
Address = Result
)
);
ClearCollect(
RequiredAttendeesNoRoom,
Concat(
RequiredAttendeesNoRoom,
Result & ";"
)
);
ClearCollect(
RequiredAttendeesNoRoom,
Split(
First(RequiredAttendeesNoRoom).Value,
";;"
)
);
Set(
RequiredAttendeesFinal,
First(RequiredAttendeesNoRoom).Result
);
Set(
RequiredAttendeesTrue,
RoomsGallery.Selected.Email & ";" & RequiredAttendeesFinal
);
Office365.V2CalendarPatchItem(
MyCalendar,
MeetingsGallery.Selected.Id,
If(
MeetingsGallery.Selected.Subject = User().FullName & "'s Skype Meeting",
User().FullName & "'s Booking",
MeetingsGallery.Selected.Subject
),
StartDateTimeUTC,
EndDateTimeUTC,
{
RequiredAttendees: RequiredAttendeesTrue,
OptionalAttendees: MeetingsGallery.Selected.OptionalAttendees,
Body: "Room has been updated to " & RoomsGallery.Selected.Name,
Location: RoomsGallery.Selected.Name,
Importance: "Normal",
ShowAs: "Busy"
}
)
);
UpdateContext({ShowLoading: false})
);
Set(
IsBooking,
false
)
Best Regards,
Bof