Code

TimeSpan start = new TimeSpan(10, 00, 00);
TimeSpan end = new TimeSpan(14, 0, 0);

if (!e.ResizeEnd)
{
      // Moving or resizing start
      e.Confirm = (e.NewStartTime.TimeOfDay >= start && e.NewStartTime.TimeOfDay <= end);
}
else
{
      // Resizing end
      e.Confirm = (e.NewEndTime.TimeOfDay >= start && e.NewEndTime.TimeOfDay <= end);
} 

Code

calendar1.ResourceViewSettings.SnapUnit = TimeUnit.Second; 

Code

private void calendar1_ItemModifying(object sender, ItemModifyConfirmEventArgs e)
{
	bool moving = !e.ResizeStart && !e.ResizeEnd;

	if (!modifying)
	{
		initialItemStartTime = e.Item.StartTime;
		initialItemEndTime = e.Item.EndTime;
		offset = TimeSpan.Zero;

		previousStartTime = e.Item.StartTime;
		previousEndTime = e.Item.EndTime;
	}

	if (moving)
		previousStartTime = e.Item.StartTime;

	if (e.ResizeStart || moving)
		offset += e.NewStartTime - previousStartTime;
	else
		offset += e.NewEndTime - previousEndTime;
	previousStartTime = e.NewStartTime;
	previousEndTime = e.NewEndTime;

	TimeSpan snapAt = new TimeSpan(12, 0, 0); // Snap at 12:00
	TimeSpan snapTollerance = new TimeSpan(3, 0, 0); // Snap when the items are within 3 hours of the snap time

	TimeSpan displace = TimeSpan.Zero;
	if (e.ResizeStart || moving)
	{
		DateTime expectedStartTime = initialItemStartTime + offset;

		// Align the time
		if (expectedStartTime.TimeOfDay >= snapAt - snapTollerance &&
			expectedStartTime.TimeOfDay <= snapAt + snapTollerance)
		{
			displace = expectedStartTime.Date + snapAt - expectedStartTime;
			expectedStartTime = expectedStartTime.Date + snapAt;
		}

		e.NewStartTime = expectedStartTime;
	}

	if (moving)
	{
		DateTime expectedEndTime = initialItemEndTime + offset + displace;
		e.NewEndTime = expectedEndTime;
	}

	if (e.ResizeEnd)
	{
		DateTime expectedEndTime = initialItemEndTime + offset;

		// Align the time
		if (expectedEndTime.TimeOfDay >= snapAt - snapTollerance &&
			expectedEndTime.TimeOfDay <= snapAt + snapTollerance)
			expectedEndTime = expectedEndTime.Date + snapAt;

		e.NewEndTime = expectedEndTime;
	}

	modifying = true;
}

private void calendar1_ItemModified(object sender, ItemModifiedEventArgs e)
{
	modifying = false;
}

private bool modifying = false;
private DateTime initialItemStartTime;
private DateTime initialItemEndTime;
private DateTime previousStartTime;
private DateTime previousEndTime;
private TimeSpan offset;