How do I compare dates in queries with datetimetz?

With Ecto-Date fields I can do query comparisons like this:

```elixir
from(e in query, where: fragment("? > NOW() - interval '24 hours'", e.some_date))
```

However that does not work.

```
operator does not exist: datetimetz > timestamp with time zone
```

I tried using Timex to generate the value to compare, but that did not work either:

```elixir
from(e in query, where: e.some_date > ^one_day_ago())

defp one_day_ago do
Timex.now()
|> Timex.shift(hours: -24)
end
```