Yes, it's been another one of these days when hours just melt into each other, and you get this growing impression that for each step forward, you go back two.
But it's not really true. I think the frustration builds up from the fact that unplanned events come into view, and lead you astray from what you originally wanted to do - or planned to do.
This happens to me on the web sometimes. Back into my academic environnement, things prop up from nowhere, people come with their problems, crashes and bugs - which often lead to other problems and bugs - and time just melts away. Suddently, it's lunchtime, and you've barely read/managed your e-mail.
Suddenly it's dark, and you've barely began what you planned - or wished - to do.
Or maybe it's just a perverse form of lack of motivation for the single task you're trying to complete, before tackling all those of last year's to do list ?