You are multiplying ints together, and overflow occurs because the maximum integer is 2^31 - 1. Only after the multiplications does it get converted to a long. Cast the first number as a long.
long days_30 = (long) 1000 * 60 * 60 * 24 * 30;
or use a long literal:
long days_30 = 1000L * 60 * 60 * 24 * 30;
That will force long math operations from the start.