getTime() in Salesforce is used to get milliseconds from a Datetime.
Sample Code:
DateTime maxTime = System.now().addMinutes(5);
DateTime minTime = System.now();
system.debug('Milliseconds Difference is ' + (maxTime.getTime() - minTime.getTime()));
system.debug('Seconds Difference is ' + (maxTime.getTime() - minTime.getTime()) / 1000);
system.debug('Minustes Difference is ' + (maxTime.getTime() - minTime.getTime()) / 60000);
Output: