@type period() ::
  {from :: DateTime.t(), to :: DateTime.t()}
  | {from :: DateTime.t(), to :: ICal.Duration.t()}

@type t() :: %ICal.Event{
  alarms: [ICal.Alarm.t()],
  attachments: [ICal.Attachment.t()],
  attendees: [String.t()],
  categories: [String.t()],
  class: String.t() | nil,
  comments: [String.t()],
  contacts: [ICal.Contact.t()],
  created: DateTime.t() | nil,
  custom_properties: ICal.custom_properties(),
  description: String.t() | nil,
  dtend: Date.t() | DateTime.t() | nil,
  dtstamp: DateTime.t() | nil,
  dtstart: Date.t() | DateTime.t() | nil,
  duration: ICal.Duration.t() | nil,
  exdates: [Date.t() | DateTime.t()],
  geo: {float(), float()} | nil,
  location: String.t() | nil,
  modified: Date.t() | nil,
  organizer: String.t() | nil,
  priority: integer() | nil,
  prodid: String.t() | nil,
  rdates: [Date.t() | DateTime.t() | period()],
  recurrence_id: Date.t() | nil,
  related_to: [String.t()],
  resources: [String.t()],
  rrule: map() | nil,
  sequence: String.t() | nil,
  status: String.t() | nil,
  summary: String.t() | nil,
  transparency: :opaque | :transparent | nil,
  uid: String.t() | nil,
  url: String.t() | nil
}