To move a chat from WhatsApp on iOS, open the Contact Info or Group Info page in WhatsApp, tap Export Chat, then choose Telegram in the Share menu.
On Android, open a WhatsApp chat, tap ⋮ > More > Export Chat, then choose Telegram in the Share menu:
WhatsApp for iOS also lets you export chats directly from the chat list. Swipe left on a chat, then choose '…' > Export Chat.
Messages will be imported into the current day but will also include their original timestamps. All members of the chat on Telegram will see the messages.