def convert_date_to_datetime(%DateTime{} = date), do: date
  def convert_date_to_datetime(%Date{} = date) do
    date
    |> Date.to_gregorian_days()
    |> Kernel.*(86400)
    |> Kernel.+(86399)
    |> DateTime.from_gregorian_seconds()
  end

value `~D[2019-05-21]` for `TransactionSchema.purchase_time` in `insert` does not match type :utc_datetime

Your processing suggests that you only want the date.

Your schema type and name suggests that you want the time as well.

If you only want the date then have a purchase_date of type :date rather than :utc_datetime.

If you want the time, then record the full date, time, and timezone.

Using a datetime type to store a date will only lead to confusion and bugs in the long run - this is especially true when timezones are involved. For example a thoughtless shift in timezone:

iex(1)> DateTime.from_iso8601("2019-05-21 00:00:00Z")
{:ok, #DateTime<2019-05-21 00:00:00Z>, 0}
iex(2)> DateTime.from_iso8601("2019-05-20 23:00:00-01:00")
{:ok, #DateTime<2019-05-21 00:00:00Z>, -3600}
iex(3)>

can lead to a different date for the same “point in time”.

Given:

iex(1)> {:ok, datetime, offset} = DateTime.from_iso8601("2019-05-23 00:00:00+03:30")
{:ok, #DateTime<2019-05-22 20:30:00Z>, 12600}

“2019-05-22 20:30:00Z” is the time that is stored in the database.

:utc_datetime is a datetime which is implicitly understood to be with reference to UTC. There is no actual timezone being stored in :utc_datetime. So when it’s retrieved you would have to DateTime.shift_zone/3 the time to the IRST timezone - which only works if you have a custom Time zone database configured. Only after the shift will it convert to the correct local date.

Otherwise:

iex(1)> {:ok, persisted_time, offset} = DateTime.from_iso8601("2019-05-22 20:30:00Z")
{:ok, #DateTime<2019-05-22 20:30:00Z>, 0}
iex(2)> DateTime.to_string(persisted_time)
"2019-05-22 20:30:00Z"
iex(3)> DateTime.to_date(persisted_time)
~D[2019-05-22]
iex(4)> 

I expect that the Date type (:date) is understood to represent a local date.