https://t.me/group_meetup_bot
(source: https://github.com/kojoru/telegram-calendar/commit/1738a0bce8d1d74b4a973608807f78355e05b6f5)

This is a mini app to share your availability with friends

Cool concept, but I wish I could choose a range of dates. Also, when I choose proposed dates, nothing happens (maybe the button is missing?)