How to only change the time in DateTime?

问题

I have a DateTime and I'm trying to do two things with it. 1: Only update the date and month. 2: Only update the time. How can I achieve that?

  DateTime currentDateTime = DateTime.now();

  void updateTime(DateTime newTime) {
       currentDateTime = ?
  }
  void updateDate(DateTime newTime) {
       currentDateTime = ?
  }

Is there any way I can destruct the currentDateTime like DateTime(...currentDateTime, ..newTime)

回答1:

Construct your Datetime object from Datetime.now() properties, instead of the whole Datetime.now() object. This should work.

  DateTime currentDateTime = DateTime.now();

  void updateTime(DateTime currentDateTime, DateTime newTime) {
       currentDateTime = DateTime(
         year: currentDateTime.year,
         month: currentDateTime.month
         day: currentDateTime.day,
         hour: newTime.hour,
         minute: newTime.minute,
         second: newTime.second,
       );
  }

  void updateDate(DateTime currentDateTime, DateTime newDate) {
       currentDateTime = DateTime(
         year: currentDateTime.year,
         month: newTime.month
         day: newTime.day,
         hour: currentDateTime.hour,
         minute: currentDateTime.minute,
         second: currentDateTime.second,
       );
  }

Remember the constructor of Datetime looks like this

DateTime(int year, [int month = 1, int day = 1, int hour = 0, int minute = 0, int second = 0, int millisecond = 0, int microsecond = 0])

回答2:

To only update time you can use TimeOfDay. This is approach that I can do

DateTime currentDateTime = DateTime.now();

void updateTime(TimeOfDay newTime) {
  // 2020-09-07 09:03:24.469
  TimeOfDay time = TimeOfDay(hour: newTime.hour, minute: newTime.minute);
  currentDateTime = DateTime(currentDateTime.year, currentDateTime.month, currentDateTime.month, time.hour, time.minute);
  // 2020-09-09 15:00:00.000
}

void updateDate(DateTime newTime) {
  // 2020-09-09 15:00:00.000
  currentDateTime = DateTime(newTime.year, newTime.month, newTime.day);
  // 2021-01-01 00:00:00.000
}

I hope this is helpful.

来源：https://stackoverflow.com/questions/63770437/how-to-only-change-the-time-in-datetime

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