I usually monitor the clearance delivery frequency in my local airport and most of the time like flights to Phoenix, Las Vegas, they issue whats termed as a "Flow Window" and causes hours of delays. And plus, the controllers give a time like "wheels-up at 34" "push back at 44" and "time now 19:55" or something like that.
I do not know what this means actually. What is a Flow time or Flow Window which causes delays and stuff???