We got our workaround working because with javascript we have complete control over the date calculation and field rendering but it is more work. We couldn’t use formula fields because we needed the conditional rendering to occur as soon as the date is entered or changed and we did not want to save (the possibly new record yet) and then re-query the record from Salesforce to get the re-calculated formula field. Apart from the delay it can also have undesirable side effects with other trigger or workflow logic fired on the record insert/update which we didn’t want.
