You can use this MS excel spreadsheet to calculate your IGT. 
Just type in your IGT result (which is shown on the points screen after every 'Phase') in this format = MM:SS,MS (Minutes:Seconds, Milliseconds) or just the seconds & milliseconds. 

Example:

01:20,34
or
00:80,34

It will always calculate right in the summary.

IGT Calculator

PilotPanda

Mobile Suit Gundam Seed Destiny: Rengou vs. Z.A.F.T. II Plus

PlayStation 2

sn0w_mountain

Koma_plus

JPN / NTSC

Route A

Route B

Route C

Route D

Route E

Route F

Route G

Route H

Route I

Easy (1)

Normal (4)

Hard (8)

Max Stats

Normal Stats

Route

Difficulty

Options

Gundam

New Timing Method

Explanation of categories option settings for arcade mode

Routing PLUS mode