All graph points show timestamp of 12/31/1899

For any graph display all points plot in a vertical bar in the center of the graph. Y values are correct but all X values (time) plot as 1:00:00 and the mouseover shows 12/31/1899. The computed values used for the time value seem OK, appearing at the expected 5 minute collection intervals, here are a few:

select unix_timestamp(substring(ts_min,1,16)) from global_query_review_history where ts_min > "2014-05-01 16:50:30"

1398984900.000000
1398984960.000000
1398984960.000000
1398985200.000000
1398985200.000000

Timezone is set in /etc/php.ini:
date.timezone =('America/Denver')

I did apply the suggested modification to the table and the collection script to speed up report generation, but that didn't have any effect on these values.

Sorry, I posted the wrong graph_defaults, should be:

$conf['graph_defaults'] = array(
'fact-group' => 'minute_ts',
'fact-order' => 'minute_ts',
'fact-limit' => '',
'dimension-ts_min_start' => date("Y-m-d H:i:s", strtotime( '-7 day')),
'dimension-ts_min_end' => date("Y-m-d H:i:s"),
'table_fields' => array('minute_ts'),
// hack ... fix is to make query builder select the group and order fields,
// then table fields only has to contain the plot_field
'plot_field' => 'Query_time_sum',
);
Latest code, Centos 6.4, PHP 5.3.3