Hi @Liridona ,
I did a test for you.
1\ Try this formula on this button control.
Set(DateSelected, Today());
Set(DateTimeRoomChange, true);
/*If(Mod(Minute(Now()), 30) <= 14,
Set(StartDateTime, DateAdd(Now(), -Mod(Minute(Now()), 30), Minutes));
Set(EndDateTime, DateAdd(StartDateTime, 30, Minutes)),
Mod(Minute(Now()), 30) >= 15,
Set(StartDateTime, DateAdd(Now(), -Mod(Minute(Now()), 30) + 30, Minutes));
Set(EndDateTime, DateAdd(StartDateTime, 30, Minutes)),
Set(StartDateTime, Now());
Set(EndDateTime, DateAdd(Now(), -Mod(Minute(Now()), 30) + 60, Minutes))
);*/
Set(StartDateTime,DateAdd(Today(),480,Minutes));
Set(EndDateTime,DateAdd(Today(),720,Minutes));
Set(StartDateTimeUTC, Text(DateAdd(StartDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z");
Set(EndDateTimeUTC, Text(DateAdd(EndDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z");
Set(BookForMeeting, false);
If(NoRoomsList,
Navigate(RoomSelectScreen, Cover),
Navigate(RoomListSelectScreen, Cover)
)

2\ Add 3 button control and set their onselect property to:
Morning button control:
Set(StartDateTime, DateAdd(DateSelected, 480, Minutes));
Set(EndDateTime, DateAdd(DateSelected, 720, Minutes));
Set(StartDateTimeUTC, Text(DateAdd(StartDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z");
Set(EndDateTimeUTC, Text(DateAdd(EndDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z")
Afternoon button control:
Set(StartDateTime, DateAdd(DateSelected, 720, Minutes));
Set(EndDateTime, DateAdd(DateSelected, 960, Minutes));
Set(StartDateTimeUTC, Text(DateAdd(StartDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z");
Set(EndDateTimeUTC, Text(DateAdd(EndDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z")
Whole day button control:
Set(StartDateTime, DateAdd(DateSelected, 480, Minutes));
Set(EndDateTime, DateAdd(DateSelected, 960, Minutes));
Set(StartDateTimeUTC, Text(DateAdd(StartDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z");
Set(EndDateTimeUTC, Text(DateAdd(EndDateTime, TimeZoneOffset(), Minutes), "[$-en-US]yyyy-mm-ddThh:mm") & ":00.000Z")

3\ Set the DisplayMode property like picture:

Best Regards,
Wearsky
If my post helps, then please consider Accept it as the solution to help others. Thanks.