How can I determine down time from logs in database table

How can I determine down time from logs in database table

I have a database table which holds error logs reported from an application.
If certain errors occur, the application requires human intervention
before becoming active again.
I need to sort through the logs and determine the total amount of time
that has accrued between every pair of events.
So when the app goes into ERROR state where intervention is required at a
certain time, I need to find the elapsed time to the next error log where
the app was restarted.
Then I need to the sum of the total elapsed time between every pair of
events.
The table looks like this:
ErrorID | ErrorMessage | ErrorDateTime
---------------------------------------------
20 | ex. msg 1 | 2013-09-01 00:10:10
21 | ex. msg 2 | 2013-09-01 00:10:15
22 | ex. msg 3 | 2013-09-01 00:10:20
23 | ERROR | 2013-09-01 00:10:25
24 | ex. msg 4 | 2013-09-01 00:10:30
25 | ex. msg 5 | 2013-09-01 00:10:35
26 | ex. msg 6 | 2013-09-01 00:10:37
27 | App Restarted | 2013-09-01 00:11:30
28 | ex. msg 7 | 2013-09-01 00:11:35
29 | ex. msg 8 | 2013-09-01 00:11:40
30 | ex. msg 9 | 2013-09-01 00:11:43
31 | ERROR | 2013-09-01 00:11:45
32 | ex. msg 10 | 2013-09-01 00:12:10
33 | ex. msg 11 | 2013-09-01 00:12:20
34 | ex. msg 12 | 2013-09-01 00:12:22
35 | App Restarted | 2013-09-01 00:13:30
So basically I need to find the difference between the timestamps of every
ERROR and the subsequent App Restarted log message.
Then get the sum of all of these durations
Can anyone point me in the right direction?