Make DateTime.parse() more permissive, accept single digit parts.

[ScottS2017]

I looked at doing a PR on this but there seem to be multiple places it could be addressed and I'm not sure which would be appropriate. So, here it is...

SDK Version:
I'm not sure which version of Dart is actually being used because Flutter is utilizing its own copy of date_time.dart located at flutter\bin\cache\pkg\sky_engine\lib\core , which I believe can, at times, be a version behind the Dart SDK. My Flutter version is 1.2.1 and I'm on Beta.

Problem:
DateTime.parse() fails with an Invalid date format error if you hand it a string that is only one character (it doesn't have leading zeros in the month, day, hour and minute for any value 9 and under).

Compounding Problem:
toString() or string interpolation drops leading zeros. IE:

print(selectedDateTime); // Returns 2019-04-18 06:03:00.000
print(selectedDateTime.hour.toString()); // Returns 6, not 06

Running this code will recreate the issue:

  DateTime testDateTime = DateTime(2019, 4, 18, 6, 5);
  print(testDateTime); // Correctly Prints 2019-04-18 06:03

  print(testDateTime.month.toString()); // Prints 4
  print(testDateTime.day.toString()); // Prints 18
  print(testDateTime.hour.toString()); // Prints 6
  print(testDateTime.minute.toString()); // Prints 5

  DateTime result = DateTime.parse('2019-04-18 06:05'); // Works
  print(result); // Correctly Prints 2019-04-18 06:03

  /// The below results in this Invalid date format error:
  // [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: FormatException: Invalid date format
  // 2019-4-18-6-5
  DateTime reconstructedDateTime = DateTime.parse(
  '2019-${testDateTime.month.toString()}-${testDateTime.day.toString()}-'
  '${testDateTime.hour.toString()}-${testDateTime.minute.toString()}');

A hacky workaround is to use a pad( ), but it would be much better if this were handled in the framework.

Example:
${_datePicked.month.toString().padLeft(2, "0")}

Thanks,
Scott Stoll

[kevmoo]

@lrhn ?

[lrhn]

The input to DateTime.parse has format requirements.
Here you are building a string which doesn't have the required format and passing it to DateTime.parse.

We could loosen those requirements, perhaps by allowing single-digit month and day or <4 digit year, but I'd prefer not to. Since we allow omitting - characters, we could only allow that if there is a -, and it would complicate the parsing for very little gain. The more permissive we are, the more errors we allow to pass through. You shouldn't be building a string from integers to pass to DateTime.parse at all.

Creating a string from integers, and then immediately parsing it, is not required, or even the best way, to create a DateTime from those integers. You should instead use:

var reconstructedTime = DateTime(testDateTime.year, testDateTime.month, testDateTime.day, testDateTime.hour, testDateTime.minute);

I'd even recommend creating the DateTime object and using its toString over building a string manually.

If you want to format a date/time in a particular way, you can use package:intl, which has a date formatter.

(That testDateTime.hour.toString() prints "6" instead of "06" is not something we can change because testDateTime.hour is an int.)

[ScottS2017]

The reason I filed it as an issue is simply because if you use DateTime.(something).toString( ) to pull something out of DateTime and then try to put it back together by using the parse, it causes an error. Intuitively, since DateTime.parse( ) takes a string, and you got strings out of the DateTime, one expects that reassembling them using parse( ) to be successful.

It might help if this simply had a different error message. Instead of:
Unhandled Exception: FormatException: Invalid date format
perhaps something like this might be more descriptive?
Unhandled Exception: FormatException: Month, Date, Hour and Minute all require two-digit integers

[derekpitts28]

Bump on this.

[lrhn]

Still no plan to change the parse behavior.

The DateTime.parse method exists to parse the output of DateTime.toString, it's not intended as a general way to parse any possible way to represent a date.
For that, use package:intl.