You have a few options.
#1) You can set up a report with a column using text mode, and a valueexpreesion similar to:
#2) You can create a calculated field that is applied to each issue you want the data on, with a similar expression as above, slightly modified: DATEDIFF(Entry Date,$$TODAY)
One thing to note if you go the field route is that custom data is not calculated every time you view it, only when it's changed, or another custom field from the same form on the same object it's on is changed.
Ex: If I update a different custom field on the same form on my issue, it'll update all the custom data on that field on that issue.