There is not an email tag for showing the transaction date. Since the thank you email is sent on the date of the transaction, this has never come up as a need before. I guess it could be implemented for other emails in the system though. We'll consider adding this.
I'd like to second this for a future development. We have mocked up an invoice that gets included in the confirmation email - and to have a field/tag for the transaction date would be most useful.