how can I set a max value on my html5 date input so that it could be recognized by safari for iOS?

Safari does not yet support it yet . Only Opera and chrome support it as of Nov 2011. Anyways here is the w3spec on it http://www.w3.org/TR/html-markup/input.date.html#input.date.attrs.max

You can use js validation on submit or on change. Use a js validation framework of your choice

Here is pure js example http://www.roseindia.net/answers/viewqa/Ajax/4734-start-date-and-end-date-validation-in-javascript.html. Use your max date instead of end date

For jquery solution look at this Validate that end date is greater than start date with jQuery