If you are working with Ecto.DateTime in your Phoenix application you may make a comparison of two variables at some point:

d1 = {{2015, 11, 30}, {0, 0, 0}} |> Ecto.DateTime.from_erl d2 = {{2015, 11, 29}, {0, 0, 0}} |> Ecto.DateTime.from_erl assert d1 > d2

The above ends up passing the assertion, but only by coincidence. What if you had a situation where you wanted to assert a comparison between today and tomorrow, but tomorrow ends up being a different month:

d1 = {{2015, 12, 1}, {0, 0, 0}} |> Ecto.DateTime.from_erl d2 = {{2015, 11, 30}, {0, 0, 0}} |> Ecto.DateTime.from_erl assert d1 > d2

The above assertion fails. If you think it seems odd that December 1, 2015 would be less than November 30, 2015 you’d be correct. To understand why we have to see what Ecto.DateTime.from_erl/1 returns:

{{2015, 12, 1}, {0, 0, 0}} |> Ecto.DateTime.from_erl # => #Ecto.DateTime<2015-11-30T00:00:00Z>

This is an Elixir Struct. The properties of the struct are not ordered, so the comparison does not actually understand the structure of datetime and how to compare properly. In this case it appears that the day values are being compared before the month values, resulting in a false assertion. To better understand this we need to take a look at the Erlang documentation for Maps (which are just Structs):

Maps are ordered by size, two maps with the same size are compared by keys in ascending term order and then by values in key order. In maps key order integers types are considered less than floats types.

To do a proper datetime comparison between two Ecto.DateTime structs we have to convert to a tuple. We can do this by using Ecto.DateTime.to_erl :

d1 = #Ecto.DateTime<2015-12-01T00:00:00Z> d2 = #Ecto.DateTime<2015-11-30T00:00:00Z> assert Ecto.DateTime.to_erl(d1) > Ecto.DateTime.to_erl(d2)

This can be cumbersome to write all the time. Thankfully Ecto comes with a nice Ecto.DateTime.compare/2 function:

d1 = #Ecto.DateTime<2015-12-01T00:00:00Z> d2 = #Ecto.DateTime<2015-11-30T00:00:00Z> assert Ecto.DateTime.compare(d1, d2) == :gt