Date function on calculated item

[pabloadmin]

Hi, i need recollect the LastCollectionEndTime on a Java JVM, but the value is a timestamp and i need covert it to human date.
I try this formula on a calculated item, but i don't know if possible use date() in this context:

date()-last(jmx[java.lang:type=GarbageCollector,name=MarkSweepComp act][LastCollectionEndTime])

I got this error: ..."cannot evaluate function"...

thank's.