I overheard this post title on switching to a frequency. No one has ever said that to me and I'm not certain which of two possible but contradictory meanings it might have. If I received it, I would ask a clarifying question, but the pilot who received it did not.
Before departing on an IFR flight, a pilot first files a flight plan, specifying for the air traffic control system exactly which route she intends to follow, with what timing. The flight plan may be filed online, by phone, or sometimes by radio. I usually file mine by phone, because I can do that while using my eyes and the non-phone-holding hand for other things, and can say what I'm doing in words rather than having to code it for the online form. I would only file by radio if I discovered while airborne a need to convert to an IFR flight, and I was out of cell tower range so I couldn't use my awesome Bluetooth headset-iPhone combo. (I didn't want Bluetooth at the time I bought the headset, but I needed a new headset RIGHT AWAY and that was all they had in stock. If you ever have reasons to talk to someone on the phone with your engine(s) running, get the Bluetooth. The days of holding the phone against your ear and yelling, "I can't hear you: I'm in the plane" are over).
The IFR flight plan is supposed to be filed minimum thirty minutes before departure, but the controllers are generally so awesome that the plan is coded and available in the system in minutes. When my company says, "Can we go to Fort Dead Royalty now?" and the flight requires an IFR clearance, I say "sure," file the flight plan for thirty minutes hence, and then board, start, run up and call for clearance. My actual take off time will usually be fifteen to twenty minutes after filing, and I'm calling for clearance as few as five minutes after filing, but it's almost always ready as soon as I need it.
The clearance I get may be identical to the one I filed, or it may be altered to comply with local procedures I didn't know about, specific runways that are active, or I may be cleared not to my destination airport but to an intermediate point, requireing me to get a new clearance before I leave that point. (Yes, I would have to stop and hold, flying in a little oval, if I hadn't received the clearance by then, but it's almost always given to me before I need it). If the airport I will be departing is a controlled airport, then when I am ready to take off, the tower controller will talk on the phone to the IFR departure/en route controllers and get an IFR release, allowing them to clear me for take off. But if the airport is uncontrolled: unstaffed or staffed only by a Flight Service Specialist, the initial IFR clearance will include a clearance cancelled time. The controller giving me the clearance is going to ensure there is a space in the system for me, but if I'm not airborne by the clearance cancelled time, I'm not allowed to go, because my space will have passed by. My departure clearance will have been cancelled. Sometimes there is also a clearance valid time, meaning that I can depart any time between the clearance valid and clearance cancelled time.
So if I were given a clearance cancelled time, and then my clearance cancelled time were cancelled, my first guess is that that means the traffic that was going to be a problem after the clearance cancelled time is no longer considered to be interfering with me--perhaps it has changed course or altitude--and that I can delay as long as I like before take off. My second guess is that "your clearance cancelled time is cancelled" is synonymous with "your clearance is cancelled." I'm ninety-five per cent sure that the first meaning is correct, but because there is a chance that I'm wrong, I'd ask. Beats getting run over by a Boeing 737.