something is wrong with that particular ATC unit.
we have no such problems.
50 % or more SLOTS we consider it as false. On example, slots for Switzerland is only considered if it is winter,in summer we are not bored with it. When you are ready -taxi and take off clearance follows.
If we apply SLOTS than, rule should be read as : -5+10 from any other applicable time ,on example, CTOT 1200 minus 5 could be anything started with 11 (1101-1159) or plus 10 anything between 1201-1259.
Third, our ACC has some system that must be cheated. So you have to give than estimate for poitns. But that estimate could be anything because, system need any info for starting process , as soon as radars catch the target -system automatically gives all actual estimates. On example a/c has slot for 1200, but via telephone you give logic estimate for that slot,regardless actual time. So 2/3/5 /10 minutes is always possible to arrange that a/c is airborne and everyone happy. We always could say, wind, eg....
