Just be aware that this might result in an invalid datetime. Depending on the timezone there might be gaps or overlaps.

If your logic allows it, it might be worth looking at the add function of the DateTime module.

date = ~D[2005-04-05]
time = ~T[08:30:00]

with_timezone = date |> Date.to_erl |> Timex.DateTime.Helpers.construct("America/Sao_Paulo") 
with_timezone_and_time = Timex.add(with_timezone, Timex.Duration.from_time(time))

This isn’t as simple as using Map.put or the %{struct | field: val} syntax, but perhaps makes things more clear.

Another possibility still:

date = ~D[2005-04-05]
time = ~T[08:30:00]
{:ok, naive} = NaiveDateTime.new(date, time)
{:ok, with_timezone_and_time} = DateTime.from_naive(naive, "America/Sao_Paulo")