According to the source, the executors are members of "Ansar al-Sharia". The date is unknown and location also. It might be the desert of Sirte, that's what I got from the comments.
Ansar al-Sharia was formed during the Libyan civil war and rose to prominence after the assassination of Muammar Gaddafi.
They are also responsible for the 2012 U.S. Consulate attack in Benghazi, that killed the US ambassador in Libya.
LiveLeak.com - VERY GRAPHIC - Libyan man executed in desert with DShK machine gun