SOLVED. YA-Playhouse where time moves slowly [s]

[Clackamas]

This is a young adult novel about twin (I think) brothers. One is an athlete and one is bookish. The bookish one gets a crush on a girl who likes the athletic one. He decides to spend the night in a playhouse where time moves at a rate on approx one year for every night. He works out the whole time trying to beef up to win the girl.

[Krista the Krazy Kataloguer]

Hi! Could it be Singularity by William Sleator?
Here's the plot summary from Worldcat.org:
"Sixteen-year-old twins Harry and Barry stumble across a gateway to another universe, where a distortion in time and space causes a dramatic change in their competitive relationship." I think the playhouse was owned by their uncle. I read it a long time ago and seem to recall that one twin aged more than the other, so that they weren't really twins any more.

[Clackamas]

That was it! Thank you Krista. A copy for my 11-year-old son is now on order =o)

[Rainbowheart]

Singularity for the link.