When you reply to your email digest, your reply gets recorded under the subject date of the digest email. It is not recorded under the date you received the digest email or under the date you sent the email.
For example, if you receive a digest email on Sept. 2 with the subject, "Here's what your team got done yesterday, Sept. 1," your reply to that email will be recorded under Sept. 1 in your iDoneThis calendar, regardless of when you send it.