t.Sub(u) would be -20 milliseconds.
The right computation is u.Sub(t), but rewrite to be even clearer.

Thanks to Karsten Weiss for catching this.

Change-Id: I6e274d69b0301840d57c5c65bf4114da0d33bf10
Reviewed-on: https://go-review.googlesource.com/46971
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>