Cell from 00:00:00 to 23:59:59 and not 12:00:00 to 12:00:00

You are probably using the sample project from the Hotel tutorial:

https://code.daypilot.org/27453/html5-hotel-room-booking-javascript-php

That project uses a manually generated time scale to adjust the cell start and end to match the check-in and check-out time.

Just use dp.scale = "Day" instead of dp.scale = "Manual" if you want to disable it.

It's set to Day, put it to Manual and back to Day and no change.

If it's set to "Day" then it will generate day cells from 00:00 to 24:00, take a look at this demo:
http://javascript.daypilot.org/demo/scheduler/scaledays.html

If you still have the problem, can you post a sample config that reproduces the issue?

I just download demo from the hotel room booking again. Didn't change anything, but cells are still from 12 to 12, even when they are set to "Day".

The hotel room booking project overrides the .scale value in loadTimeline() method:

function loadTimeline(date) {
  dp.scale = "Manual";
  dp.timeline = [];
  var start = date.getDatePart().addHours(12);

  for (var i = 0; i < dp.days; i++) {
      dp.timeline.push({start: start.addDays(i), end: start.addDays(i+1)});
  }
  dp.update();
}

You need to change it there like this:

function loadTimeline(date) {
  dp.scale = "Day";
  dp.startDate = date;
  dp.update();
}

Thank you! Its working

And ..is there anything like copy and paste cell on right click, button, etc? Or drag and copy without option?

No worries, I already figured it out. Made Copy button on edit page.

You can also implement copy & paste using context menu. It's described here:
https://doc.daypilot.org/scheduler/event-copying/