the problem with this line:
const unsigned long postingInterval = 60*1000; // delay between updates, in millisecond
is that the two numbers (60 and 1000) are by default defined as int constants. For AVRs ints have the size of 16-bit, so the multiplication between two ints with a result that will exceed the maximum dimension for the type, generate an overflow.
The solution should be the variable cast that you propose or assigning the constant with a different type, like:
const unsigned long postingInterval = 60L*1000; // Long typ e constant
or
const unsigned long postingInterval = 60000; //assigning directly
This example will be fixed in the next IDE release.
Thanks for reporting!