Calculates how many years I've been married to Julia :-)

yearsmarried.php

<?php 
function guru_yearsmarried() {
    
    // dates to be calculated
    $dateMarried = strtotime( '2004-04-22 15:00:00' );
    $currentDate = strtotime( current_time ( 'mysql' ));

    // calculate the time in seconds and convert
    $seconds = $currentDate - $dateMarried;
    $years = $seconds / 60 / 60 / 24 / 365.242199;

    return round ($years);
}

// register shortcode
function guru_shortcodes() {
    add_shortcode( 'yearsmarried', 'guru_yearsmarried' );
}
add_action( 'init', 'guru_shortcodes' );