IE8 object doesn't support this property or method

IE8 object doesn't support this property or method

I am building web apps for a company and it had worked in IE9, Firefox and chrome no problem.
Using IE8 (most of our departments use) I get an error "object doesn't support this property or method".

The line it's showing me is:

Var index = arrDates.indexOf( currDate );

There is a plugin of a calendar which shows up and when you click on a date it's supposed to pop up an input field so you can type. That input doesn't show up and I get the above error message only in IE8.

Also some other web apps using jquery in IE8 receive the same error.

We have built it on jquery 1.8.2

I tried using jquery 1.11.1 but no luck.

Upgrading everyone to IE9 is not an option.

We need IE8 to work. Any help?