const Duration({int days: 0, int hours: 0, int minutes: 0, int seconds: 0, int milliseconds: 0, int microseconds: 0 })

Creates a new Duration object whose value is the sum of all individual parts.

Individual parts can be larger than the next-bigger unit. For example, hours can be greater than 23.

All individual parts are allowed to be negative. All arguments are 0 by default.

Source 

const Duration(
    {int days: 0,
    int hours: 0,
    int minutes: 0,
    int seconds: 0,
    int milliseconds: 0,
    int microseconds: 0})
    : this._microseconds(MICROSECONDS_PER_DAY * days +
          MICROSECONDS_PER_HOUR * hours +
          MICROSECONDS_PER_MINUTE * minutes +
          MICROSECONDS_PER_SECOND * seconds +
          MICROSECONDS_PER_MILLISECOND * milliseconds +
          microseconds);