The group did not say what would happen if the deadline was not met or when the countdown begins.
"Jason" said: "Today is November 18."
"I have been here now for 173 days and I feel we have been forgotten."
No other hostage was shown in the video.
Britain's foreign office condemned the broadcast, saying it "serves only to add to the distress of the men's families and friends".
British officials have not released the names of the kidnapped men, and have requested their identities not be disclosed by the media.
The kidnapping took place May 29, when about 40 gunmen in police uniforms and driving vehicles used by Iraqi security forces kidnapped four security contractors and a computer consultant from an Iraqi finance ministry compound.
The video was posted as Britain prepares to hand over security control of oil-rich Basra province to the Iraqis in mid-December.
British troops withdrew in September from their last base in Basra city to an airport garrison on the outskirts.