https://pub.dev/packages/supercharged посмотри
var duration = 5.minutes + 30.seconds;
duration += 0.5.hours;
var future = 2.days.fromNow() + 20.minutes;
var past = 15.minutes.ago();
var allDaysOf2020 = DateTime(2020).until(DateTime(2021));
var allFullHoursOfFirstJanuary2020 =
DateTime(2020, 1, 1).until(DateTime(2020, 1, 2), by: 1.hours);
await 2.seconds.delay; // waits for 2 s