Date Subtraction

I have two periods in time from a Cisco router that I would like to find the difference in seconds. I am not sure the best way to do this since it is not a date, but rather an amount of time since last reset. Here is the numbers 181 days, 7:11:06.66 //stands for 181 days, 7 hours, 11 minutes, 6.66 seconds. The next is 181 days, 7:16:6.75 //stands for 181 days, 7 hours, 16 minutes, 7.75 seconds. I would probably shave off the .66 and .75 seconds while it was still a string. It may be faster to round when it's in seconds, I really don't know. Then what do I do? I was thinking of using strtotime(); but because it's not really a date, I am at a loss for what to do. Any help with this would be great.