Wrong results for DIFF when using a custom timezone

[jul1ans]

When I calculate the diff between two dates with a custom timezone, there are wrong results when I use days format.
This only happens if start and end date were in different summer / winter timezones (but I want to force it to calculate the diff like if there is no summer / winter time)

Here is a dummy code:
https://codepen.io/jul1ans/pen/LYbegrp?editors=0010

When I use the hours diff and calculate the days manually, it works correct.
Math.floor(totalHours / 24)

My specs:

Thu Feb 25 2021 14:07:59 GMT+0100
25.2.2021, 14:07:59
-60
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.182 Safari/537.36
2.29.1

Thank you in advance