PHP Cookbook

3.6. Finding the Difference of Two Dates with Julian Days

Problem

You want to find the difference of two dates measured by what a clock would say, not the actual elapsed time.

Solution

Use gregoriantojd( ) to get the Julian day for a set of date parts, then subtract one Julian day from the other to find the date difference. Then convert the time parts to seconds and subtract one from the other to find the time difference. If the time difference is less than 0, decrease the date difference by one and adjust the time difference to apply to the previous day. Here’s the code:

$diff_date = gregoriantojd($date_1_mo, $date_1_dy, $date_1_yr) -
             gregoriantojd($date_2_mo, $date_2_dy, $date_2_yr);
$diff_time = $date_1_hr * 3600 + $date_1_mn * 60 + $date_1_sc -
             $date_2_hr * 3600 - $date_2_mn * 60 - $date_2_sc;
if ($diff_time < 0) {
   $diff_date--; 
   $diff_time = 86400 - $diff_time;
}

Discussion

Finding differences with Julian days lets you operate outside the range of epoch seconds and also accounts for DST differences.

If you have the components of your two days in arrays:

// 7:32:56 pm on May 10, 1965 list($date_1_yr, $date_1_mo, $date_1_dy, $date_1_hr, $date_1_mn, $date_1_sc)= array(1965, 5, 10, 19, 32, 56); // 4:29:11 am on November 20, 1962 list($date_2_yr, $date_2_mo, $date_2_dy, $date_2_hr, $date_2_mn, $date_2_sc)= array(1962, 11, 20, 4, 29, 11); $diff_date = gregoriantojd($date_1_mo, $date_1_dy, $date_1_yr) - gregoriantojd($date_2_mo, $date_2_dy, $date_2_yr); $diff_time = $date_1_hr * 3600 ...