send email with past date

I am looking for help with sending an email with a past date. I will be using Gmail for the email platform. The purpose of sending the email with a past date is for saving my life. A document needed to get sent that day, and it will literaly save my sanity and life. I need help to ensure that the email will be sent from the correct date and time with a particular date. Hopefully, this will help to correct any errors or for verification purposes. An attachment will be sent with the email 8 pages long, text-only and no images or pictures. Experience in accessing gmail account is an asset. I do not have much time left. Thank you