Difference between steady_clock vs system_clock?

I am trying to see whether my data is 120 second old or not by looking at the timestamp of the data so I have below code:

uint64_t now = duration_cast<milliseconds>(steady_clock::now().time_since_epoch()).count();
bool is_old = (120 * 1000 < (now - data_holder->getTimestamp()));

In the above code data_holder->getTimestamp() is uint64_t which returns timestamp in milliseconds.

Now when I print out now variable value, I see this 10011360 and when I print out data_holder->getTimestamp() value which is 1437520382241 so the difference of now and data holder timestamp should be negative right? Why it is coming as positive as shown in the below logs?

2015-07-21 16:13:02,530 WARN 0x7f35312d1700 data_check - now value: 10011360 , data holder timestamp: 1437520382241 , difference: 18446742636199180735

Does my above code looks right? And from the above data holder timestamp, it doesn't look to be 120 second old data right so I feel something is wrong in my code? Since if I convert that data holder timestamp to actual time (using epoch converter) and then compare it with logs time as shown above it is almost same.

I am using steady_clock as shown above. Do I need to use system_clock here? What is the difference between steady_clock vs system_clock in layman terms. I am running this code on Ubuntu 14.04 box.