Flutter : DateTime.now().toIso8601String() Math

You can parse string to DateTime object:

final dateTime = DateTime.now();
final stringDateTime = dateTime.toIso8601String();
final parsedDateTime = DateTime.parse(stringDateTime);

In this case, dateTime and parseDateTime are the same. Then to find out the time difference use difference DateTime instance method, which returns Duration instance. Example from dart documentation:

final berlinWallFell = DateTime.utc(1989, DateTime.november, 9);
final dDay = DateTime.utc(1944, DateTime.june, 6);

final difference = berlinWallFell.difference(dDay);
print(difference.inDays); // 16592

Use the difference function on DateTime object:

  DateTime now = DateTime.now();
  DateTime addedThreeDays = DateTime.now().add(Duration(days: 3));
  
  print(3 >= now.difference(addedThreeDays).abs().inDays);