TAT means Turn around time.. For e.g. Dakshin arrives HYB around 5 in the morning and departs at around 11:30 in the night. that means TAT is nearly 18 hours.. so even if comes late by 10 hours in the worst case, it will have 8 hours time for maintenance. So, it can start its journey towards Nizamuddin without any delay. I hope I have cleared your doubt.