At my home airport, BCN we have had many problems with ATC due to slot times.
For example two weeks ago a B742 that was parked in a remote stand that required push-back procedure missed their slot and then got a big big one.
The problem is the following:
The aircraft had its flight plan updated for 1415. Then there are 15 extra minutes in which the pilot can call the ATC and receive push-back clearance if they DO NOT have any CTOT.
But that day we had a CTOT for 1430. The pilot called the ATC at 1415 and push-back clearance was given so they started the push-back. As it is a 742, it takes at least 3 minutes for standard push-back and then another 5 minutes for the rest of engines that have not been previously turned on. Total time for the push-back is then 8 minutes. The aircraft was then ready to taxi to the runway at 1423, 7 minutes before the CTOT.
Usual Taxi Time at BCN is 20 minutes and so is stated at the NOTAMs (or that's what we have been told).
The aircraft arrived at 1438 at the runway and when they contacted ATC they were told that they had missed the CTOT. The crew asked for a delay and they got a CTOT for 2 hours later.
Then my question is, if the Taxi Time is 20 minutes why that aircraft was cleared to be pushed later than the "20 minutes before CTOT" time? Is this a ATC mistake?