Hmm. If you chalk up the Zohar as a practical joke played on the Jewish community, you pretty much have to discount all of the Kabbalistic works, which I'm not ready to do.
As for "when it was actually composed", our degree of accuracy is roughly in line with most religious works of unknown origin -- i.e., "within about 1000 years". It was written no later than the 13th century, and very unlikely to be written before the 1st century CE. It stands out in that most books that we can't date any more precisely than that are far older than (possibly) 13th century works, but still -- I don't think it's any more dubious than half of the Tanakh or most of the Christian mystic writings, or the Christian Gospels.
(Something I've noticed -- generally the Jewish writings have less certainty about the time of their composition, but greater certainty that the contents haven't been significantly altered since they were written, compared to the Christian writings, which are far more accurately dated, but also far more likely to have been "adjusted" along the course of their lifetime.)
no subject
As for "when it was actually composed", our degree of accuracy is roughly in line with most religious works of unknown origin -- i.e., "within about 1000 years". It was written no later than the 13th century, and very unlikely to be written before the 1st century CE. It stands out in that most books that we can't date any more precisely than that are far older than (possibly) 13th century works, but still -- I don't think it's any more dubious than half of the Tanakh or most of the Christian mystic writings, or the Christian Gospels.
(Something I've noticed -- generally the Jewish writings have less certainty about the time of their composition, but greater certainty that the contents haven't been significantly altered since they were written, compared to the Christian writings, which are far more accurately dated, but also far more likely to have been "adjusted" along the course of their lifetime.)