A check if two date or time ranges A and B overlap needs to cover a lot of cases:

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1. A partially overlaps B

2. A surrounds B

3. B surrounds A

4. A occurs entirely after B

5. B occurs entirely after A

This means you actually have to check that:

• neither does A occur entirely after B (meaning A.start > B.end)
• nor does B occur entirely after A (meaning B.start > A.end)

Flipping this, A and B overlap iff A.start <= B.end && B.start <= A.end

The code below shows how to implement this in Ruby on Rails. The example is a class Interval, which has two attributes #start_date and #end_date. These dates are considered inclusive, meaning that if one interval ends on the day another interval starts, we consider those two intervals overlapping.

Note how we implemented both a version for loaded Ruby objects (Interval#overlaps?) and a scope that returns all overlapping intervals (Interval.overlapping). Depending on your problem you might need one or both or those.

class Interval < ActiveRecord::Base

  validates_presence_of :start_date, :end_date

  # Check if a given interval overlaps this interval    
  def overlaps?(other)
    start_date <= other.end_date && other.start_date <= end_date
  end

  # Return a scope for all interval overlapping the given interval, excluding the given interval itself
  scope :overlapping, -> { |interval|
    where("id <> ? AND start_date <= ? AND ? <= end_date", interval.id, interval.end_date, interval.start_date)
  }

end

This assumes that start_date <= end_date is always true.

Mixing dates and times

If you mix dates and times incomparisons, mind to not compare datetimes with date ranges in MySQL.

Visualisation

1. ------- ------- 
   AAA       AAA
    BBB     BBB
   ------- -------

2. -------
   AAA
    B
   -------
   
3. -------
    A
   BBB 
   -------
   
4. -------
      AAA
   BBB
   -------
   
5. -------
   AAA
      BBB
   ------