You can add to the Jot custom field and you can apply to it some sense of the date, but a full-fledged date behavior is impossible. The Jot stores additional (custom) fields in a separate table where all these fields are kept in textual format (name of the custom field may be up to 50 characters long and its value is of type text which means 64 KB chunk of data). Full-fledged date behavior needs a field to be the date itself or the so-called unixstamp.
Provided that your users fill in your custom date field in strict format, YYYY-MM-DD, it will serve well for intended sorting, but you need, of course, to force your users in some way for strict format of their input. You can validate field with a regular expression
&validate=`MyDate:^\d\d\d\d-\d\d-\d\d$`
Above is a very basic suggestion for such a validation. The better your regex is the stricter format your users will be forced to.
The detailed description of Jot’s parameters you can find at
http://wiki.modxcms.com/index.php/Jot