One of the new features in Apple's Mail app in Lion (Mac OS X 10.7) is a widescreen-friendly, three-column layout, displaying your mailboxes on the left, your message list in the middle, and a message preview on the right. I love this layout--I used a Mail add-on to get it under Snow Leopard (OS X 10.6)--but it's not without drawbacks.
One of those is that there's less room to display information about each message in the message list. As a result, Mail shortens the date display dramatically compared to what you'd see in the "classic" Mail layout. Specifically, for messages received today, Lion Mail shows only the time received; for messages received yesterday, Mail displays just the word Yesterday; and for messages received prior to yesterday, Mail displays just the date.
Macworld reader Lynn P. wanted to know if there's a way to get Mail to display both the date and time for all messages. Though I haven't found a way to display the date and time for messages received yesterday--Mail is downright determined to use just Yesterday for those messages--with a bit of trickery, it's possible to add the date for messages received today, and to add the time to messages received prior to yesterday.
For messages received today or prior to yesterday, Mail uses your preferences for time and date format, respectively, in the Language & Text pane of System Preferences. Here's how to tweak those settings to get today's messages to also display the date, and older messages to also display the time.
Open the Language & Text pane of System Preferences. Click the Formats tab. The Dates format--specifically, the Short format in the Customize dialog for Dates--is what Mail uses for days prior to yesterday. The Times format--specifically, the Short format in the Customize dialog for Times--is what Mail uses for today. But you'll notice that time options aren't listed in the Dates dialog, and date options aren't listed in the Times dialog.
Here's the trick: Click Customize for Dates. Set up your preferred date format in the Short field. Select everything in that field (Edit: Select All) and copy it (Edit: Copy). Click Cancel to exit the Date-format dialog without saving your changes. Click Customize for Times, and then set up your preferred time format in the Short field. Now put the cursor at the end of the Short-format field, press the right-arrow key (to be sure nothing is selected), type a comma (,), then paste (Edit: Paste) the date-format you copied earlier. The result should look something like 12 : 00 AM , 1 / 5 / 11.
The format is now fixed for messages received today, but you still need to fix it for older messages. Select the entire Short field's contents and, again, copy (Edit: Copy), then click OK. Click Customize for Dates. Delete everything in the Short-format field, and then paste (Edit: Paste) what you copied from the Times dialog. Click OK.
You should now have the same Short format, showing both the time and date, in both the Times and Dates dialogs. Mail's message list should immediately reflect these changes, displaying both the time and date for all messages (except those received yesterday).
Note that these changes also affect the display of the Short date and time in other places throughout OS X--for example, the Date Modified and Date Created columns in Finder windows. If you ever decide to go back to the original date and time formats, you can just open the Customize window for date and time and, in the Short field, delete the capsules for time and date, respectively.