editing human_time_diff()

Unfortunately human_time_diff doesn’t have a filter that you can hook into to provide what you’re looking for. The best thing you can probably do is copy the function code itself into a new function and use that instead:

<?php

function custom_human_time_diff( $from, $to = '' ) {
	if ( empty( $to ) )
		$to = time();

	$diff = (int) abs( $to - $from );

	if ( $diff < 15 * MINUTE_IN_SECONDS ) {
		$since = __( 'a few minutes ago' );
	}  elseif ( $diff < HOUR_IN_SECONDS ) {
		$since = __( 'a half hour ago' );
	} elseif ( $diff < DAY_IN_SECONDS && $diff >= HOUR_IN_SECONDS ) {
		$hours = round( $diff / HOUR_IN_SECONDS );
		if ( $hours <= 1 )
			$hours = 1;
		$since = sprintf( _n( '%s hour', '%s hours', $hours ), $hours );
	} elseif ( $diff < WEEK_IN_SECONDS && $diff >= DAY_IN_SECONDS ) {
		$days = round( $diff / DAY_IN_SECONDS );
		if ( $days <= 1 )
			$days = 1;
		$since = sprintf( _n( '%s day', '%s days', $days ), $days );
	} elseif ( $diff < 30 * DAY_IN_SECONDS && $diff >= WEEK_IN_SECONDS ) {
		$weeks = round( $diff / WEEK_IN_SECONDS );
		if ( $weeks <= 1 )
			$weeks = 1;
		$since = sprintf( _n( '%s week', '%s weeks', $weeks ), $weeks );
	} elseif ( $diff < YEAR_IN_SECONDS && $diff >= 30 * DAY_IN_SECONDS ) {
		$months = round( $diff / ( 30 * DAY_IN_SECONDS ) );
		if ( $months <= 1 )
			$months = 1;
		$since = sprintf( _n( '%s month', '%s months', $months ), $months );
	} elseif ( $diff >= YEAR_IN_SECONDS ) {
		$years = round( $diff / YEAR_IN_SECONDS );
		if ( $years <= 1 )
			$years = 1;
		$since = sprintf( _n( '%s year', '%s years', $years ), $years );
	}

	return $since;
}

I’ve tried to meet your requirements with the above, but you might need to make modifications to cover the case of 16 – 29 minutes (which you didn’t explicitly mention in your original post).

thank you so much for your help!