DateTime Delay

gistfile1.dart

import 'dart:async';

void main() {
  
  print('${DateTime.now()} - First');
  executeOnMinute(() => print('${DateTime.now()} - Hello'));
}

void executeOnMinute(void Function() callback) {
  DateTime now = DateTime.now();
  DateTime nextHourToSalutate;
  
  if((now.hour >= 8) && now.hour < 12){
    nextHourToSalutate = DateTime(
      now.year,
      now.month,
      now.day,
      12,
      0,
      0,
    );
  } else if(now.hour >= 12 && now.hour < 19){
    nextHourToSalutate = DateTime(
      now.year,
      now.month,
      now.day,
      19,
      0,
      0,
    );
  } else {
    nextHourToSalutate = DateTime(
      now.year,
      now.month,
      now.day + 1,
      8,
      0,
      0,
    );
  }
  
  Future.delayed(nextHourToSalutate.difference(now), () {
      callback.call();
    executeOnMinute(callback);
 });
}