I have three different IMAP e-mail providers. Here is what I have observed. Looking at the sent messages in each provider's server on the iPad Air the sent time shows differently for each one. The Exchange Server at work shows the sent mail as Time (sent today), followed by Day of Week for seven days, followed by sent date. My ISP shows Today, Yesterday, followed by Day(s) of week for seven days, followed by the sent date(s). A third e-mail provider I use shows Day(s) of Week for seven days, followed by sent date(s). I did not compare these to my MacBookAir Mail which has the same providers and accounts.
It appears it depends on your provider and how the IMAP mail dates are configured to appear on an iPad.
This was just a cursory examination since I send e-mails using three different providers. It might be something you can adjust on their mail web site.
Have a nice day.