Gah! Here's what I got when I looked up "how to calculate GMT for Chicago" :
Find the number of longitude lines between you and the Prime Meridian in Greenwich, England. Remember when you were in school learning geography, and looking at the maps with those lines running north and south, from pole to pole? Those are longitude lines and they are used in calculating coordinates for travel and also for calculating time, relative to the Prime Meridian. Count the number of north/south longitude lines between you and the Prime Meridian. Between the Prime Meridian in England, and Chicago, Illinois, there are six longitude, or meridian lines. Determine if you're east or west of the Prime Meridian. Look at the world map again. If you're west of the Prime Meridian, your GMT will be ahead of, or minus, the time at the Prime Meridian. If you are east, your time will be after, or plus, GMT. Put the minus or plus sign in front of the number you found from the previous step and that's your GMT.
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Find the number of longitude lines between you and the Prime Meridian in Greenwich, England. Remember when you were in school learning geography, and looking at the maps with those lines running north and south, from pole to pole? Those are longitude lines and they are used in calculating coordinates for travel and also for calculating time, relative to the Prime Meridian. Count the number of north/south longitude lines between you and the Prime Meridian. Between the Prime Meridian in England, and Chicago, Illinois, there are six longitude, or meridian lines. Determine if you're east or west of the Prime Meridian.
Look at the world map again. If you're west of the Prime Meridian, your GMT will be ahead of, or minus, the time at the Prime Meridian. If you are east, your time will be after, or plus, GMT. Put the minus or plus sign in front of the number you found from the previous step and that's your GMT.