1) Identifier 'DISPLAY_CALENDAR' must be declared.
This looks like your trigger is calling a procedure DISPLAY_CALENDAR that either doesn't exist, or perhaps is defined in a library that you haven't attached.
2) too many declarations of 'TO_CHAR' match this call
This probably means you have applied a TO_CHAR to a value that is already a string. TO_CHAR can be called for dates or numbers, so it tries to helpfully coerce your string into something appropriate but doesn't know which version you want - date or number. You may not need the TO_CHAR call at all, or if you do you may need to explicitly convert the argument first like this:
TO_CHAR( TO_NUMBER(:input), '999.99')
TO_CHAR( TO_DATE(:input, 'DD-MON-YYYY' /* current format */), 'YYYY-MM-DD' /* desired format */)