It's often more appropriate to return a date difference in a human readable string. We've posted a version of WordPress' human_time_diff() 
function here (to be used outside of WP). However, there are countless means of achieving the same result. The following is a function we also regularly use.
The function accepts any strtotime() 
compatible time string.
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<?php
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/*
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Human Time Difference
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http://www.beliefmedia.com/code/php-snippets/human-time-difference
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See also: http://www.beliefmedia.com/readable-time-difference
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*/
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function beliefmedia_time_difference($date) {
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/* If no date provided */
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if (empty($date)) return "No date provided";
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$periods = array('second', 'minute', 'hour', 'day', 'week', 'month', 'year', 'decade');
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$lengths = array('60', '60', '24', '7', '4.35', '12', '10');
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/* The two timestamps */
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/* Check validity of date */
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if (empty($unix_date)) return "Bad date";
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/* Determine tense of date */
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if ($now > $unix_date) {
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$difference = $now - $unix_date;
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$tense = 'ago';
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} else {
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$difference = $unix_date - $now;
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$tense = 'from now';
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}
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$difference /= $lengths[$j];
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}
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if ($difference != 1) $periods[$j] .= 's';
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return "$difference $periods[$j] {$tense}";
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}
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/* Usage */
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$date = "2017-06-20 11:00am";
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echo beliefmedia_time_difference($date);
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?>
See also our WP snippet: Display Time To Published or Scheduled Posts.
