In Java, the way you write constants define their types. When you write "86400/7", the compiler understands that the two numbers are integer and the result should be integer also, so instead of getting "12342.85714285714" into "birth" you are getting only "12342".
Try to write your constants differently: instead of 86400, use 86400.0 or 86400d.
Take a look here, in the topic "Floating-point literals".
Hope this helps.