I replied with, "the museum opens at 9:00 am." This too was not accepted, but I thought it would make the exception given that I put "am" to indicate when. I guess the museum can also open at night as well. Plus, I realized too late that it doesnt recognize commas, quotations etc
For the 24-hour system, the expression is "at nine hundred hourS" or, for short, "at nine hours", "at fourteen", etc. So here, 'at the hours (time) of nine'. 'alle'.
To other complainers, i.e. not the OP with a fair enquiry, please embrace and accept Italian -and other language systems. They all have features that are as valid as or more logical than the one(s) you know. Let's learn Italian here.