Trouble formatting date in ISO 8601 format

I want to display post dates in ISO 8601 format, like this: 2007-08-20.

In my story template, I tried "$yr-$mo_num-$da", but that doesn't
work. All I get out is the day number (20).

If I put spaces around the dashes, "$yr - $mo_num - $da", it works
fine -- except that I have spaces around the dashes in the output:
2007 - 08 - 20.

What's going on here? Is there a way to generate the date without
spaces around the dashes?

Thanks,
Steve

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