New answers tagged conventions
Just use the text "Today". As a "badge" style is no harm either. e.g. download bmml source – Wireframes created with Balsamiq Mockups The principle is always to match the users mental model.
A reasonable compromise would be to have the button not highlighted (have a neutral background color perhaps) when it's on the off state, and highlight it (change background color) when it's in the on state.
There is also this piece from 2004 (by the looks): "Eyetrack III: What News Websites Look Like Through Readers’ Eyes". http://www.poynter.org/uncategorized/24963/eyetrack-iii-what-news-websites-look-like-through-readers-eyes/
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