Widget tarihSec(){
  return   Padding(
    padding: EdgeInsets.only(left: 20, right: 20, bottom: 15),
    child: Material(
      elevation: 5,
      shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.all(
            Radius.circular(15.0),
          )),
      child: TextFormField(
        style: TextStyle(fontSize: 15, fontWeight: FontWeight.bold),
        decoration: InputDecoration(
          icon: Icon(Icons.date_range),
          labelText: "Tarih seçiniz...",
          border: InputBorder.none,
          labelStyle: TextStyle(fontSize: 15),
          contentPadding:
          const EdgeInsets.symmetric(vertical: 5.0, horizontal: 10.0),
        ),
        controller: _dogumTar,
        onTap: () {
          // Below line stops keyboard from appearing
          FocusScope.of(context).requestFocus(new FocusNode());

          // _showDatePicker(context);
          _showDatePickerNormal();
        },
      ),
    ),
  );
}
  late TextEditingController _dogumTar = new TextEditingController();
  String secilenTarih = "";
  void _showDatePickerNormal() {
    showDatePicker(
        context: context,
        initialDate: secilenTarih == ""
            ? DateTime.now()
            : DateTime.parse(getTarChange()),
        firstDate: DateTime.now(),
        lastDate: DateTime.now().add(Duration(days: 365)))
        .then((value) {
      if (value != null) {
        secilenTarih = formatDate(value!, [dd, '.', mm, '.', yyyy]);

        _dogumTar.text = secilenTarih;
        if (secilenTarih == "") {
          secilenTarih = formatDate(DateTime.now(), [dd, '.', mm, '.', yyyy]);
        }

        setState(() {
          _dogumTar.text = secilenTarih;
        });
        // Navigator.of(context).pop();

      }
    });
  }

String getTarChange() {

    if (secilenTarih == "") {

      return formatDate(DateTime.now(), [yyyy, '-', mm, '-', dd]);

    }

    String yil = secilenTarih.split(".")[2].toString();

    String ay = secilenTarih.split(".")[1].toString();

    String gun = secilenTarih.split(".")[0].toString();

    return "${yil}-${ay}-${gun} 11:47:00";

  }

2-) hazır widget olarak kullanımı

       1. selectdater.dart

        import 'package:date_format/date_format.dart';
import 'package:flutter/material.dart';

class SelectDateR extends StatefulWidget {
  String secilenTarih = "";
  String title = "Seçiniz...";
  Function secilenClick;
  bool bugunSeciliGelsin = false;

  SelectDateR({
    required this.secilenTarih,
    required this.title,
    required this.secilenClick,
    required this.bugunSeciliGelsin,
  });

  @override
  State<SelectDateR> createState() => _SelectDateRState();
}

class _SelectDateRState extends State<SelectDateR> {
  List<String> items = [];
  String secilenTarih = "";
  String title = "";
  late Function secilenClick;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    this.secilenTarih = widget.secilenTarih;
    this.title = widget.title;
    this.secilenClick = widget.secilenClick;

    if(widget.bugunSeciliGelsin){
      secilenTarih= formatDate(DateTime.now(), [dd, '.', mm, '.', yyyy]);
    }

    controller = new TextEditingController(text: secilenTarih);
  }

  final TextEditingController textEditingController = TextEditingController();

  @override
  void dispose() {
    textEditingController.dispose();
    super.dispose();
  }

  late TextEditingController controller;

  Widget dateContent() {
    return Container(
      padding: EdgeInsets.only(top: 10, left: 10),
      width: 175,
      height: 50,
      child: TextFormField(
        decoration: InputDecoration(
          labelText: title,
          border: OutlineInputBorder(),
        ),
        controller: controller,
        onTap: () {
          // Below line stops keyboard from appearing
          FocusScope.of(context).requestFocus(new FocusNode());

          // _showDatePicker(context);
          tarihShow();
        },
      ),
    );
  }

  String getTarChange(String secilenTarih) {
    if (secilenTarih == "") {
      return formatDate(DateTime.now(), [yyyy, '-', mm, '-', dd]);
    }
    String yil = secilenTarih.split(".")[2].toString();
    String ay = secilenTarih.split(".")[1].toString();
    String gun = secilenTarih.split(".")[0].toString();
    return "${yil}-${ay}-${gun} 11:47:00";
  }

  void tarihShow() {
    showDatePicker(
            context: context,
            initialDate: secilenTarih == ""
                ? DateTime.now()
                : DateTime.parse(getTarChange(secilenTarih)),
            firstDate: DateTime.now().add(Duration(days: -100000)),
            lastDate: DateTime.now().add(Duration(days: 365)))
        .then((value) {
      if (value != null) {
        secilenTarih = formatDate(value!, [dd, '.', mm, '.', yyyy]);

        controller.text = secilenTarih;
        if (secilenTarih == "") {
          secilenTarih = formatDate(DateTime.now(), [dd, '.', mm, '.', yyyy]);
        }

        setState(() {
          controller.text = secilenTarih;
          secilenClick(secilenTarih!);
        });
        // Navigator.of(context).pop();

      }
    });
  }

  @override
  Widget build(BuildContext context) {
    // DropdownButtonHideUnderline
    return Center(
      child: dateContent(),
    );
  }
}

      2. KULLANIMI

    SelectDateR(
  secilenClick: (String secilenBasTar) {
    print(secilenBasTar);
  },
  secilenTarih: "",
  title: "Baş Tar",
  bugunSeciliGelsin: true,
),