Fix invalid String#indexOf test in a coercible Date instance

Fixes #793

As described by @ediosyncratic:

Date(0) flows through the given algorithm to ToDateString, which should:

> Return an implementation-dependent String value that represents tv as
> a date and time in the current time zone using a convenient, human-readable form.

The string is implementation-dependent and there is nothing constraining it to use
a two-digit day-of-month (so "Thu Jan 1, 1970, 0:0:0" would be valid), much less to
position it at index 8.