clojure - best way to increase date by x days - clojure

Question

I need to read in a date string in yyyyMMdd format and increase it by x amount of days - at the minute I am doing it by converting to millis and adding 1 day in mills then converting back to yyyyMMdd.
(.print
(.withZone
(DateTimeFormat/forPattern "yyyymmdd") (DateTimeZone/forID "EST"))
(+ 86400000 (.parseMillis
(.withZone (DateTimeFormat/forPattern "yyyymmdd")
(DateTimeZone/forID "EST")) "20150401")))
Is there a cleaner way to do this? clj-time library is not available to me, and I am using clojure 1.2

Answer 1

Since you cant't use clj-time, which is the best option in this case, I can't think of anything better than using org.joda.time as you did.
However, I would suggest rewriting your code a little bit:
there is no need for time zones here;
you could create DateTimeFormat object once and reuse it.
Here is how your function could look:
(defn add [date pattern days]
(let [fmt (DateTimeFormat/forPattern pattern)
add (* 86400000 days)]
(->> date
(.parseMillis fmt)
(+ add)
(.print fmt))))
(add "20150401" "yyyymmdd" 1) ; => "20150402"
If you don't want to work with milliseconds, you could use .parseDateTime instead of .parseMillis and .plusDays method do add days to the parsed date:
(defn add [date pattern days]
(let [fmt (DateTimeFormat/forPattern pattern)
dt (.parseDateTime fmt date)]
(.print fmt (.plusDays dt days))))