I applied for a shengen visa through Czech Republic embassy, the intended date of flight was 24/12, I booked without paying the flights but she said it’s too early and that I can’t get the visa on that time, she kept the flights reservations with me and took everything else and said that she expect me to get the visa on 27th, my question is..How can that be possible when I write the intended date of entry on the application as the 24th, can they approve the visa after the flight date?!!
They can approve the visa any time they like, but only for (then) future travels.
The people examining your visa application and either denying or granting it, doesn't really care when you want to go. They care more (and are paid to do so) about denying/granting visas to the right people. And that takes time, so when you were told you could get the visa on the 27th, that would have been based on how busy they are.
If you get a visa on the 27th, and the proccessing clerk cares (see Neusser's comment) it should not be valid from the 24th but probably from the 27th and the number of days you asked for, but it's probably more likely that they'll contact you to hear when you would like a visa.