Other than perhaps a lot of buried work. Everything from wire conduit to concrete takes time. Additionally if they are actually building a stage that pops out of the ground I'm sure that takes some additional time to make sure everything is set perfectly.
The thing is, if you have worked in the construction industry for any length of time you understand that little problems can really screw with a construction schedule. You do your best to make up the time but sometimes its just an unavoidable part of the process. Back when i was doing concrete work i seen jobs delayed for 2 weeks because of one significant rain storm. I've also seen multi day delays because the building inspectors were behind schedule. Other delays could be for a material shortage or even your coworker having to take a few days off because the kids are sick. The point I'm trying to make here is you have no idea as to why there is a delay in the construction and arbitrarily placing a completion date (before the permit expiration date) on the project is completely ridiculous.