Compute if trim timestamp to start from not 1970, the max year same length base 36 zzz... can present to.

base36-timestamp-trimmed.php

<?php
// Compute if trim timestamp to start from not 1970, the max year same length
// base 36 zzz... can present to.

$startYear = 2000;
$currentYear = date('Y');

$currentTimestamp = time();
for ($year = $startYear; $year <= $currentYear; $year ++) {
    $startDate = "$year-01-01";
    echo "Start date: " . $startDate;

    // Compute relative timestamp
    $startDateTimestamp = strtotime($startDate);
    $timestamp = $currentTimestamp - $startDateTimestamp;
    $encoded = base_convert($timestamp, 10, 36);
    $length = strlen($encoded);
    echo ", encoded: $encoded, $length bytes";

    // Compute max relative date by this length
    $maxTimestamp = $startDateTimestamp +
        base_convert(str_repeat('z', $length), 36, 10);
    $maxDate = date('Y-m-d H:i:s', $maxTimestamp);
    echo ", max to $maxDate";

    echo PHP_EOL;
}