Subtracting date variables?

[rdp9090]

I'm having trouble displaying the correct dates on the variable expression I set. On 1 variable, I have the expression 

new SimpleDateFormat("MM/dd").format(new Date()) 

which correctly displays the current date, and on another variable, 

 new SimpleDateFormat("MM/dd").format(new Date().getTime() - 7* 24 * 60 * 60 * 1000)

which correctly displays the date 7 days ago. However, when I try  

 new SimpleDateFormat("MM/dd").format(new Date().getTime() - 30* 24 * 60 * 60 * 1000)

The date goes up into 07/19, which is 20 days forward rather than 30 days backwards. Could anyone explain why this occurs, and how I could get the correct date out of this?

 

[Friendly User]

Possible the equation 30* 24 * 60 * 60 * 1000 overflows because that value is way to big for an integer to store. Might want to try something like 30L * 24 * 60 * 60 * 1000 so it forces to a Long.