这是一个时间日期提醒，提示输入的日期必须是未来的日期。解决办法是，在代码中检查输入的日期是否在当前日期之前，如果是，那么就提示用户输入未来日期。下面是一个示例代码片段：

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

class MyPage extends StatefulWidget {
  @override
  _MyPageState createState() => _MyPageState();
}

class _MyPageState extends State {
  final _formKey = GlobalKey();
  late TZDateTime _selectedDate;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Select a future date')),
      body: Center(
        child: Form(
          key: _formKey,
          child: Column(
            children: [
              Padding(
                padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
                child: TextFormField(
                  decoration: InputDecoration(labelText: 'Select a date'),
                  onTap: () async {
                    final date = await showDatePicker(
                      context: context,
                      initialDate: TZDateTime.now(local),
                      firstDate: TZDateTime.now(local),
                      lastDate: TZDateTime.utc(2100),
                    );
                    if (date != null) {
                      _selectedDate = TZDateTime.from(date, local);
                      if (_selectedDate.isBefore(TZDateTime.now(local))) {
                        // The selected date is in the past
                        ScaffoldMessenger.of(context).showSnackBar(
                          SnackBar(content: Text('Please select a future date')),
                        );
                      }
                    }
                  }
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}