Who can solve this for me........... PLEASE ???

[Westlife]

Ron and Dave have just spotted red hot sexy Noi in The Up To You Bar and want to Bar-Fine her .But Noi can't decide which one to go with so she tells them " if they can tell me when my birthday is I will go with you  for FREE  "   

For a clue she gives them 10 possible dates !!

May 15th, 16th or 19th
June 17th or 18th
July 14th or 16th
August 14, 15th or 17th

Noi whisper's in Ron and Dave ear's separately the Month and the day of her birthday respectively! But tells them they can only have one guess or no deal 

Ron..... I don’t know when Noi’s birthday is, but I know that Dave doesn’t know too.

Dave.... At first I don’t know when Noi’s birthday is, but now I know.

Ron.... Then I also know when Noi’s birthday is.

I am in the Up to You Bar too and Noi has told me that as Ron and Dave are taking so long to come up with the answer  she is willing to offer the same deal to me   

So... can  somebody tell me ( ASAP ) the answer so I can get Noi back to my Room for a freebie before Ron and Dave can figure out the answer 

[Westlife]

As you lot are a bit slow I found this 

 https://www.youtube.com/watch?v=kNG0khN1vN4

And Noi got a good Westing