On the newer and older version of Gmail, but not in the basic HTML version, in the upper right corner, next to Settings, a link appeared labeled, “New! Gmail Custom Time”.

The page claimed a new Gmail feature had been added that would allow people to date stamp their emails in the past, allowing for all sorts of mischief. Users would be limited to ten Custom Times a year to minimize the number of fraudulent emails.
You’d probably ask why exacly ten. Their researchers have found out that allowing each person more than ten pre-dated emails per year would cause people to lose faith in the accuracy of time, thus rendering the feature useless.

Their findings:

N = Total emails sent
P = Probability that user believes the time stamp
φ = The Golden Ratio
L = Average life expectancy

With GMail Custom Time, you can date your email in the past. Just click “Set custom time” from the Compose view - any email you send to the past appears in the proper chronological order in your recipient’s inbox. You can opt for it to show up read or unread by selecting the appropriate option.

The link led to a 404 error before April 1st. - It now leads to their latest hoax, Gmail Custom Time.

Clicking any of the three links at the bottom of the page brought the user to a page indicating that Gmail Custom time was, in fact, their April Fools prank for 2008.