Description:
------------
In PHP Manual
›Supported Date and Time Formats
›Relative Formats

number format is
[+-]?[0-9]+
, no decimal point included.

If number is given with decimal value, number should be cut before decimal point, eg "+1.5" should be read as "+1"
Now, it is read as "5"




Test script:
---------------
<?php

// in examples below, this script was run at 30.10.2021

echo date("j.n.Y", strtotime("+1.5 year", time()));
echo date("j.n.Y", strtotime("-1.5 year", time()));

?>


Expected result:
----------------
30.10.2022
// 1 year ahead

30.10.2020
// 1 year backwards

Actual result:
--------------
30.10.2026
// This is 5 years ahead, not 1.5 years ahead

30.10.2026
// This is 5 years ahead, not 1.5 years backwards