An example of how the DatePicker extension for jTelegramBotAPI can be used.

MyAwesomeBot.java

import com.jtelegram.api.TelegramBotRegistry;
import com.jtelegram.api.chat.id.ChatId;
import com.jtelegram.api.commands.filters.MentionFilter;
import com.jtelegram.api.commands.filters.TextFilter;
import com.jtelegram.api.menu.Menu;
import com.jtelegram.api.menu.MenuHandler;
import com.jtelegram.api.menu.viewer.RegularMenuViewer;
import com.jtelegram.api.requests.message.send.SendText;
import com.jtelegram.api.update.PollingUpdateProvider;
import com.jtelegram.api.util.TextBuilder;
import com.jtelegram.ext.datepicker.DatePickerExtension;
import com.jtelegram.ext.datepicker.menu.DatePickerMenu;
import com.jtelegram.ext.datepicker.picker.DatePicker;
import com.jtelegram.ext.datepicker.picker.DatePickerOptions;
import java.time.LocalDate;
import java.util.Locale;

/**
 * @author Nick Robson
 */
public class MyAwesomeBot {

    public static void main(String[] args) {
        TelegramBotRegistry registry = TelegramBotRegistry.builder()
                .updateProvider(new PollingUpdateProvider())
                .build();
        registry.registerBot(System.getenv("BOT_TOKEN"), (bot, ex) -> {
            if (ex != null) {
                ex.printStackTrace();
                System.exit(1);
            }

            System.out.format("Started bot: @%s\n", bot.getBotInfo().getUsername());

            // when /datepicker@botusername is sent (or /datepicker is PM'd)
            bot.getCommandRegistry().registerCommand(new MentionFilter(
                    new TextFilter("datepicker", false, (event, command) -> {
                        try {
                            String localeString = command.getArgsAsText();
                            Locale locale = Locale.forLanguageTag(localeString);
                            if (localeString.isEmpty() || locale == null) {
                                locale = Locale.forLanguageTag(command.getSender().getLanguageCode());
                            }

                            // get the date picker instance corresponding to their locale
                            DatePicker datePicker = DatePickerExtension.forLocale(locale);
                            Menu menu = DatePickerMenu.builder()
                                    .bot(bot)
                                    .datePicker(datePicker)
                                    .datePickerOptions(DatePickerOptions.builder()
                                            // always highlight today's date with a • next to it
                                            .dateHighlightedPredicate(LocalDate.now()::isEqual)
                                            .build())
                                    // only the command sender can change the month
                                    .monthSelectionPredicate((cb, date) -> cb.getQuery().getFrom().getId() == command.getSender().getId())
                                    // only the command sender can change the month
                                    .dateSelectionPredicate((cb, date) -> cb.getQuery().getFrom().getId() == command.getSender().getId())
                                    // when the date is clicked...
                                    .dateSelectionConsumer((cb, date) -> {
                                        System.out.format(
                                                "@%s clicked %s\n",
                                                cb.getQuery().getFrom().getUsername(),
                                                date.toString()
                                        );
                                        bot.perform(SendText.builder()
                                                .chatId(ChatId.of(cb.getQuery().getMessage()))
                                                .replyToMessageID(cb.getQuery().getMessage().getMessageId())
                                                .text(TextBuilder.create().escaped("I like ").bold(date.toString()).escaped(" too!"))
                                                .errorHandler(Throwable::printStackTrace)
                                                .build());
                                    })
                                    // OPTIONAL, to be displayed once the menu has loaded
                                    .messageSupplier(() -> TextBuilder.create().escaped("Choose a date"))
                                    .build();

                            bot.perform(SendText.builder()
                                    .chatId(ChatId.of(event.getMessage()))
                                    .replyToMessageID(event.getMessage().getMessageId())
                                    .text(TextBuilder.create().escaped("Loading..."))
                                    .callback(tm -> {
                                        menu.addViewer(RegularMenuViewer.builder()
                                                .chatId(tm.getChat().getChatId())
                                                .messageId(tm.getMessageId())
                                                .build());
                                        MenuHandler.registerMenu(menu);
                                    })
                                    .errorHandler(Throwable::printStackTrace)
                                    .build());
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                        return true;
            })));
        });
    }

}