Two things stopped me from adding tests here:
1. The JavaScript specification doesn't specify what constitutes a valid date/time string for parsing.
2. I couldn't find any examples of wildly-flagrant date headers in the wild.

I'm not particularly wedded to the last ditch mechanism of Date.parse; it was mostly added as a preemptive attempt to avoid issues, especially given that I didn't want to reverse-engineer all of PR_Now's parsing decisions and I knew that JS_Now was half-based on it.