bag to csv

edit

I am doing this to grab a topic from my bag rostopic echo -p -b bagfile.bag /topic1 > topic1.txt

an example text file is this:

%time,field.axes0

1319232665843818430,0.0

1319232665893904494,0.0

is %time unix time? because I can't seem to convert it.

What I would like is to convert this to tell me the number of seconds from its start, so the bag starts with 0 time, this is to match the timestamp on an audiofile logged at the same time as this.

Thanks