sql sum of date differences

i am developing an android application which has a table named "entries" the table structure is as follows

id     name      operation       date
----------------------------------------------
1     engine        on        08-03-2015 19.22
2     engine        off       08-03-2015 19.23
3     light         on        08-03-2015 19.23
4     light         off       08-03-2015 19.28
5     engine        on        08-03-2015 19.28
6     engine        off       08-03-2015 19.30

i woul like to get the output as follows

name    duration
----------------
engine  3
light   5

That is the total time on which the engine and light if in on state. In detail for engine the time difference b/w on and off state are added and taken as output.similarly for light. Anyone plz help me Dont know how to start. Thanks in advance