A datetime picker with custom initial value.

custom_input_date_picker.dart

import 'package:datetime_picker_formfield/datetime_picker_formfield.dart';
import 'package:flutter/material.dart';
import 'package:intl/intl.dart' show DateFormat;

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final _formKey = GlobalKey<FormState>();
  String startDate;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: Text('Demo'),
        ),
        body: Form(
          key: _formKey,
          child: Column(
            children: <Widget>[
              BasicDateField(initialText: startDate),
              TextFormField(
                decoration: InputDecoration(
                  hintText: "Insert date here and press Enter...",
                ),
                onChanged: (val) => setState(() => startDate = val),
                onEditingComplete: () => setState(() {
                  print('onEditingComplete: "$startDate"');
                  _formKey.currentState.validate();
                }),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class BasicDateField extends StatelessWidget {
  BasicDateField({Key key, String initialText})
      : _editdatanascita = TextEditingController(text: initialText),
        super(key: key);

  final TextEditingController _editdatanascita;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        DateTimeField(
          controller: _editdatanascita,
          decoration: InputDecoration(hintText: 'Nata il'),
          format: DateFormat("dd/MM/yyyy"),
          onShowPicker: (context, currentValue) {
            return showDatePicker(
                context: context,
                firstDate: DateTime(1930),
                initialDate: currentValue ?? DateTime.now(),
                lastDate: DateTime(2080));
          },
          validator: (input) {
            print('validate: "$input"');
            return null;
          },
          onSaved: (input) => null,
        ),
      ],
    );
  }
}