Emails go into your calendar on the day that the email was originally sent.
For example, if we sent you an email on October 10 asking what you did that day, and you reply on October 11, your email will go into your calendar for October 10, not October 11.
If we sent you an email on October 11, asking what you did that that day, you can reply with what you got done on October 10. Just change the subject line, from October 11 to October 10. Whatever date you put in the subject line, that's the date for which your entry will be recorded.
Need more help? email us at firstname.lastname@example.org