[racket] srfi/19 UTC time

When I use string->date in srfi/19, it defaults to my local time zone.
I see from the documentation that one can use "~z" to denote the "time
zone in RFC-822 style". A quick google of RFC-822 suggests "UT" is the
correct abbreviation for UTC.

However

Welcome to Racket v5.3.4.
> (require srfi/19)
> (string->date "22/05/13/UT" "~d/~m/~y/~z")
string->date: TIME-ERROR type bad-date-format-string: "~d/~m/~y/~z"
  context...:
   /usr/lib/racket/collects/srfi/19/time.rkt:1532:0: tm:string->date
   /usr/lib/racket/collects/srfi/19/time.rkt:1569:0: string->date
   /usr/lib/racket/collects/racket/private/misc.rkt:87:7

So in short: how - given the day, month and year - can I construct a
date object that's in UTC?