What if you can't start a new job right away?

So I sent my resume off to this job at least a month ago. I just had an interview with them today. It's just a PT job for the weekends. The thing is I won't be able to work the first 2 weekends in April because I had prearranged vacation plans. I haven't been offered the job yet but I just hope that won't deter them from hiring me. I filled out an official application today and when it asked "when can you start" I wrote April 16th (but can train earlier). I should have written down my vacation dates but I didn't and i'm not sure when or if I should mention it to them. I haven't been offered the job, still waiting on word. Should I call the manager and let them know or should I wait to see if I get offered the job and then let them know? I really want the job and I don't want to screw it up because I can't work start right away.....I'm probably just being paranoid :-(