How can a startup manage its product release cycles?

As a startup, we've limited human resources who must perform multiple operations. I wish to understand from fellow startup folks what are their hacks to manage product release cycle? 

We seem to miss the deadlines by a few days almost every time. Upon some retrospective analysis, it looks like we do not factor in the buffer times for unexpected issues. 

We wanted to release our much awaited 'Downloads' app today; but have missed the deadline. A few nasty bugs popped up at the end-moment that pushed our release further. 

Would really appreciate responses. Thank you in advance.