Steps to Reproduce

1. Create date custom field and fill the value in all issues.
2. Search any Issues and save in filter.
3. Add Time Since Chart gadget and choose date custom field.
4. Get error in log (see Capture001.png):

   2012-12-07 21:42:13,581 http-8083-3 ERROR admin 1302x560x1 16rvdbh 0:0:0:0:0:0:0:1 /rest/gadget/1.0/timeSince/generate [common.error.jersey.ThrowableExceptionMapper] Uncaught exception thrown by REST service
   java.lang.NullPointerException
   	at java.util.TreeMap.getEntry(TreeMap.java:324)
   	at java.util.TreeMap.get(TreeMap.java:255)