GoogleFinace convert dd/mm/yy to yy,mm,dd format

It actually works if you just add the dates as they are without the date function wrapped around them

=min(GoogleFinance("HKDUSD","price",L1-1,L1))

Edited: To answer your follow up question about the date format - a formulaic workaround you do uses regex replace and basically just swaps the two capture groups in month and day with this regex "(\w+)/(\w+)":

=min(GoogleFinance("HKDUSD","price",REGEXREPLACE(L1,"(\w+)/(\w+)","$2/$1")-1,REGEXREPLACE(L1,"(\w+)/(\w+)","$2/$1")))