Are the dates really that old? You may need to increase MAX_DAYS_AGO. Does it work differently for more recent timestamps?
http://docs.splunk.com/Documentation/Splunk/6.4.2/Admin/Propsconf
MAX_DAYS_AGO = <integer>
* Specifies the maximum number of days past, from the current date, that an
extracted date can be valid. Splunk still indexes events with dates older
than MAX_DAYS_AGO with the timestamp of the last acceptable event. If no
such acceptable event exists, new events with timestamps older than MAX_DAYS_AGO
will use the current timestamp.
* For example, if MAX_DAYS_AGO = 10, Splunk applies the timestamp of the last
acceptable event to events with extracted timestamps older than 10 days in
the past. If no acceptable event exists, Splunk applies the current timestamp.
* Defaults to 2000 (days), maximum 10951.
* IMPORTANT: If your data is older than 2000 days, increase this setting.
