I've taken my things to Diane's from a coat alteration to jeans. I forget how much they charged me for the coat (maybe $26 or something?) but pants hemming is just $10 plus tax.
They would first tell you the date it will be done (usually about a week), but if you ask them if they could rush it a bit and have something by certain date (I did this once when I was going on a trip the following week), they will work with you.
I've been satisfied with their work thus far, and they have dressing rooms, so you can take the shoes you'll wear with the dress and have them pin it to match the height.
P.S. They also have a person doing alterations at Village Cleaners in the U Village Mall, and I know she does formal dress alterations as well. The turnaround time may be slower, however. They said she's busy because it's wedding season. She's only there M-Th.
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