The thing that has worked for me is very similar to what you are doing, with the difference being that I use the UNIX timestamp for SINCE and UNTIL.
Example:
https://graph.facebook.com/212686148747689/insights/
page_impressions_by_city_unique/week/?
access_token=QWERTYUI&since=1315699200&until=1320796800
(That's all supposed to be on one line, but it's easier to read it this way, at least for me.)
With this approach, you want to be careful and make sure that the difference between SINCE and UNTIL is not bigger that 90 days. Otherwise, you'll get an error, like so:
(#604) The specified date range cannot exceed 7776000 seconds
Finally, if you don't have a way of generating the UNIX timestamp automatically, go to a web site like:
http://www.epochconverter.com/
If anyone else has some better insights, please share. I hope this helps.