This is item #5 from Greg Hudson's mail about 2.0 API improvements:
http://subversion.tigris.org/servlets/ReadMsg?list=dev&msgNo=67095
"Our timestamp parser should eliminate support for the deprecated format."
I believe this refers to the following in libsvn_subr/time.c:
/* Our old timestamp strings looked like this:
*
* "Tue 3 Oct 2000 HH:MM:SS.UUU (day 277, dst 1, gmt_off -18000)"
*
* The idea is that they are conventionally human-readable for the
* first part, and then in parentheses comes everything else required
* to completely fill in an apr_time_exp_t: tm_yday, tm_isdst,
* and tm_gmtoff.
*
* This format is still recognized on input, for backward
* compatibility, but no longer generated.
*/
static const char * const old_timestamp_format =
"%3s %d %3s %d %02d:%02d:%02d.%06d (day %03d, dst %d, gmt_off %06d)";