use_floor_for_human_time_diff.php

<?php
function use_floor_for_human_time_diff( $since, $diff, $from, $to ) {
	if ( empty( $to ) ) {
		$to = time();
	}

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

	if ( $diff < HOUR_IN_SECONDS ) {
		$mins = floor( $diff / MINUTE_IN_SECONDS );
		if ( $mins <= 1 )
			$mins = 1;
		/* translators: Time difference between two dates, in minutes (min=minute). 1: Number of minutes */
		$since = sprintf( _n( '%s min', '%s mins', $mins ), $mins );
	} elseif ( $diff < DAY_IN_SECONDS && $diff >= HOUR_IN_SECONDS ) {
		$hours = floor( $diff / HOUR_IN_SECONDS );
		if ( $hours <= 1 )
			$hours = 1;
		/* translators: Time difference between two dates, in hours. 1: Number of hours */
		$since = sprintf( _n( '%s hour', '%s hours', $hours ), $hours );
	} elseif ( $diff < WEEK_IN_SECONDS && $diff >= DAY_IN_SECONDS ) {
		$days = floor( $diff / DAY_IN_SECONDS );
		if ( $days <= 1 )
			$days = 1;
		/* translators: Time difference between two dates, in days. 1: Number of days */
		$since = sprintf( _n( '%s day', '%s days', $days ), $days );
	} elseif ( $diff < MONTH_IN_SECONDS && $diff >= WEEK_IN_SECONDS ) {
		$weeks = floor( $diff / WEEK_IN_SECONDS );
		if ( $weeks <= 1 )
			$weeks = 1;
		/* translators: Time difference between two dates, in weeks. 1: Number of weeks */
		$since = sprintf( _n( '%s week', '%s weeks', $weeks ), $weeks );
	} elseif ( $diff < YEAR_IN_SECONDS && $diff >= MONTH_IN_SECONDS ) {
		$months = floor( $diff / MONTH_IN_SECONDS );
		if ( $months <= 1 )
			$months = 1;
		/* translators: Time difference between two dates, in months. 1: Number of months */
		$since = sprintf( _n( '%s month', '%s months', $months ), $months );
	} elseif ( $diff >= YEAR_IN_SECONDS ) {
		$years = floor( $diff / YEAR_IN_SECONDS );
		if ( $years <= 1 )
			$years = 1;
		/* translators: Time difference between two dates, in years. 1: Number of years */
		$since = sprintf( _n( '%s year', '%s years', $years ), $years );
	}

	return $since;
}

add_filter( 'human_time_diff', 'use_floor_for_human_time_diff', 10, 4 );