[patch] <th> around first row elements

I don't understand why, in the index pages, the first data element of a table data row (the date range) is enclosed in <th></th>. This appears to be for visual effect.

To my tiny mind, the date range is "just data" and should not be tagged as row-headers. I've attached the smallest patch I could make, against -2.2.0. I think it changes the markup to the right thing.

However it is nice to have a visual distinction between the date range and the threads. That could be done with CSS and a class tag.

Which leads me to ask -

What would be the right class name to use to enable CSS selection of the date range column? Multielement class tags are allowed - eg

    <td class="first daterange">
which can be selected by either

    .first { }
or

    .daterange { }

Cheers
Vince