Нашел решение:
<?php
$today = time();
$start_date = mktime(0,0,0,12, 15, 2006); # just threw in a random date
$diff = $today - $start_date;
$days = floor( $diff/86400 );
$startDay = date( 'w', $start_date );
$endDay = date( 'w', $today );
# Check to see if start is on a weekend
if ( $startDay == 6 ) # Saturday
$days -= 2;
elseif ( $startDay == 0 ) # Sunday
$days -= 1;
# Check to see if end is on a weekend
if ( $endDay == 6 ) #
Saturday
$days -= 2;
elseif ( $endDay == 0 ) # Sunday
$days -= 1;
$weeks =
$daysWithoutWeekends = floor( $days/7 ) * 5;
echo $daysWithoutWeekends;
?>