def update_existing(map, key, fun) do
  case map do
    %{^key => old} -> %{map | key => fun.(old)}
    _ -> map
  end
end

def update_indifferent(map, key, fun) when is_atom(key) do
  map
  |> update_existing(key, fun)
  |> update_existing(Atom.to_string(key), fun)
end

And then

params
|> update_indifferent(:start_time, &CalendarUtils.from_iso8601/1)
|> update_indifferent(:end_time, &CalendarUtils.from_iso8601/1)

I have a function called deep_map that I use in several different projects to simplify certain classes of map operations. On top of that I have other functions like atomize_keys/3 and so on. It might be useful (or it might be not )

Summary

Recursively traverse a map and invoke a function
that transforms the map for each key/value pair.

Arguments

• map is any t:map/0
• function is a 1-arity function or function reference that
  is called for each key/value pair of the provided map. It can
  also be a 2-tuple of the form {key_function, value_function}
  • In the case where function is a single function it will be
    called with the 2-tuple argument {key, value}
  • In the case where function is of the form {key_function, value_function}
    the key_function will be called with the argument key and the value
    function will be called with the argument value
• options is a keyword list of options. The default is []

Options

• :level indicates the starting (and optionally ending) levels of
  the map at which the function is executed. This can
  be an integer representing one level or a range
  indicating a range of levels. The default is 1..#{@max_level}
• :only is a term or list of terms or a check function. If it is a term
  or list of terms, the function is only called if the key of the
  map is equal to the term or in the list of terms. If :only is a
  check function then the check function is passed the {k, v} of
  the current branch in the map. It is expected to return a truthy
  value that if true signals that the argument function will be executed.
• :except is a term or list of terms or a check function. If it is a term
  or list of terms, the function is only called if the key of the
  map is not equal to the term or not in the list of terms. If :except is a
  check function then the check function is passed the {k, v} of
  the current branch in the map. It is expected to return a truthy
  value that if true signals that the argument function will not be executed.

Notes

If both the options :only and :except are provided then the function
is called only when a term meets both criteria.

Returns

• The map transformed by the recursive application of
  function

Examples

  iex> map = %{a: :a, b: %{c: :c}}
  iex> fun = fn
  ...>   {k, v} when is_atom(k) -> {Atom.to_string(k), v}
  ...>   other -> other
  ...> end
  iex> Cldr.Map.deep_map map, fun
  %{"a" => :a, "b" => %{"c" => :c}}
  iex> map = %{a: :a, b: %{c: :c}}
  iex> Cldr.Map.deep_map map, fun, only: :c
  %{a: :a, b: %{"c" => :c}}
  iex> Cldr.Map.deep_map map, fun, except: [:a, :b]
  %{a: :a, b: %{"c" => :c}}
  iex> Cldr.Map.deep_map map, fun, level: 2
  %{a: :a, b: %{"c" => :c}}

It is very important to point out that this should be used ONLY if you trust source of the binaries. If used on untrusted source then you can experience DoS when malicious party will send data with a lot different strings.