Time script help sought!

[kpp9c]

I am kind of in a bit of a jam  (okay a big jam) and i was hoping that
someone here could give me a quick hand. I had a few pages of time
calculations to do. So, i just started in on them typing them in my
time calculator and writing them in by hand. Now i realize, that i
really need a script to do this because:

1. It turns out there are hundreds of pages of this stuff.
2. I have to do something similar in again soon.
3. By doing it by hand i am introducing wonderful new errors!
4. It all has to be typed up anyway (which means weeks of work and even
more typos!)

The input would like so:

Item_1    TAPE_1    1    00:23    8:23

Item_2    TAPE_1    2    8:23    9:41

Item_3    TAPE_1    3    9:41    10:41
Item_3    TAPE_1    4    10:47    11:19
Item_3    TAPE_1    5    11:21    11:55
Item_3    TAPE_1    6    11:58    12:10
Item_3    TAPE_1    7    12:15    12:45    Defect in analog tape sound.
Item_3    TAPE_1    8    12:58    24:20    Defect in analog tape sound.

Item_4    TAPE_1    9    24:33
Item_4    TAPE_1    10    25:48
Item_4    TAPE_1    11    29:48
Item_4    TAPE_1    12    31:46
Item_4    TAPE_1    13    34:17        Electronic sounds.
Item_4    TAPE_1    14    35:21
Item_4    TAPE_1    15    36:06
Item_4    TAPE_1    16    37:01    37:38

These are analog tapes that were digitized (on to CD or a digital tape)
that have now been exported as individual files that are meant to be
part of an on-line audio archive. The timings refer to the time display
on the CD or digital tape. The now all have to adjusted so that each
item starts at 0.00 since they have all been edited out of their
context and are now all individual items that start at 00:00. So Item_1
which was started at 00:23 on the tape and ended at 8:23 needs to have
23 seconds subtracted to it so that it says:

Item_1    TAPE_1    1    00:00    08:00

Item_2    TAPE_1    2    08:23    09:41

would change to:

Item_2    TAPE_1    2    00:00    01:18

etc.

but as always you may notice a wrinkle.... some items have many times
(here 6) indicated:

Item_3    TAPE_1    3    9:41    10:41
Item_3    TAPE_1    4    10:47    11:19
Item_3    TAPE_1    5    11:21    11:55
Item_3    TAPE_1    6    11:58    12:10
Item_3    TAPE_1    7    12:15    12:45    Defect in analog tape sound.
Item_3    TAPE_1    8    12:58    24:20    Defect in analog tape sound.

This is all a single sound file and these separate times mark where
there was a break, defect, or edit in the individual item. These have
to be adjusted as well to show where these events would appear in the
new sound file which now starts at 00:00.

Item_3    TAPE_1    3    00:00    01:00    ----
Item_3    TAPE_1    4    01:00    01:38    ----
Item_3    TAPE_1    5    01:38    02:14    ----
Item_3    TAPE_1    6    02:14    02:29    ----
Item_3    TAPE_1    7    02:29    03:04    Defect in analog tape sound.
Item_3    TAPE_1    8    03:04    14:39    Defect in analog tape sound.

Further wrinkles: Some have start and end times indicated, some only
start times. I suppose that the output would ideally have both.... some
have comments and others don't ... and I need these comments echo-ed or
since i probably need to make a database or table eventually non
comments just have some place holder.

I'd have a lot of similar type calculations to do... I was hoping and
praying that some one here was feeling generous and show me the way and
then, of course i could modify that to do other tasks... Usually i am
happy to take the long road and all but i'll be honest, i am in a big
jam here and this huge task was just dumped on me. I am frankly a
little desperate for help on this and hoping someone is feeling up to
spoon feeding me a clear modifiable example that works. Sorry.....
cheers,

kevin