PHP Calculation 'downtime'

Question

I've made an program in PHP that stores in a MySQL database when it's working. The table that shows whether the program is working or not looks like this:

Table_name: downtime
Working(int)    Time(datetime)
1               2011-05-06 15:18:55
0               2011-05-06 15:20:21
1               2011-05-06 15:20:24
0               2011-05-06 16:05:13
1               2011-05-06 16:05:15
0               2011-05-06 16:27:59
1               2011-05-06 16:28:01
0               2011-05-06 16:37:35
1               2011-05-06 16:37:37

Now I would like to retrieve the 'downtime' between to dates. Say date 1 is 2011-05-06 15:20:24 and date 2 is 2011-05-06 16:28:15, then I would like the script to return the sum of the differences between the 1's and 0's in that period. How can I make this?

Important to know if when date 1 for example is 2011-05-06 15:20:22, then there are two extra seconds downtime. It's hard to think of a (almost) perfect solution.