Helper function to compare the value of a PLIST_DATE node against a given number of seconds since epoch (UNIX timestamp).
Parameters
datenode
node of type PLIST_DATE
cmpval
Number of seconds to compare against (UNIX timestamp)
Returns
0 if the node's date is equal to the supplied values, 1 if the node's date is greater than the supplied values, or -1 if the node's date is less than the supplied values.