One possible way to get around this is to have ecash issuers pay interest on ecash. However it requires ecash to be timestamped and therefore compromises its untraceability. (Think of the timestamp as a serial number.)
It wouldn't exactly have to be timestamped. By convention, all interest bearing currency could be denominated as of some fixed date. For instance, its future value as of Jan 1, 2200 A.D. The issuer could then pay interest without knowing the date the currency was issued. (Of course, some accounting rules are probably going to need changing, hehe) Neither the payee nor the issuer needs to know the actual issue date when settlement time comes. When you buy a t-bill, it is worth some amount on some date. You don't know when the previous owner bought it or how much (s)he paid. The denominated date could even vary if it were "blinded". As long as the present value of the ecoin is the same, the issuing institution should not care how it is expressed. A variable interest rate scheme could even prevent an announced fixed rate from conveying clues about the issue date. Rick F. Hoselton (who doesn't claim to present opinions for others)