The operating system of RevPi Core is a (customized) Linux based operating system from RaspberryPi foundation. Those operating systems are called, for example, Wheezy or Jessie.
To query the software version and its installation date proceed as follows:
cat /etc/revpi/image-release 2017-05-24-revpi-jessie.img
In this case it is a Jessie operating system fron 24th May 2017.