Hi, i'm trying to format a date with slashes but I can't. I can use any separator exept slashes
no, it is not a bug :-)
The slash is being interpreted by the FormatDateTime routine as a formatting character, you expected it to be literal.
Whenever this routine (and accompanied routines) detect a special formatting character, then that formatting character will be replaced as described in the link given below.
How exactly the replacement turns out in the end depends on your (global) system-settings. It can have a different outcome depending on the end-user system settings. It is locale dependent.
Try to put your slashes between quotes.
See:
https://www.freepascal.org/docs-html/rtl/sysutils/formatchars.html