How to set the Firebird date format or why doesn't dd/mm/yyyy work?
In general, you shouldn't ever be using the strings user entered directly in your queries. Convert the string into some kind of Date object and pass it to Firebird as query parameter.
If you still wish to use strings, here are the formats that Firebird supports:
In all of them, year can be written with 2 or 4 digits. The same format is also used in timestamps.
Additionally, you can use the format where months are represented by 3-character name (JAN,FEB,MAR...) instead of number (01,02,03...):