Hi @Örnvall Andreas
I tried to reproduce your site, a Team site with default language in English, region is set to Swedish (Sweden).
The date format is YYYY-MM-DD. This formula works perfectly on my side.
=INT(EndTime-StartTime)&"d "&INT(MOD((EndTime-StartTime)*24;24))&"h "&ROUND(MOD((EndTime-StartTime)*1440;60);0)&"m"
If you need only date (with no time), try using this official formula from Microsoft.
=DATEDIF(StartTime;EndTime;"D")&" days"