The Ads API is returning INVALID_TIME_WINDOW for one particular date, but not others


I have found that if I make a call to the Ads API with the start_time parameter set to “2013-10-20” then the API returns a message that says

Expected start_time to be precisely aligned at midnight in timezone America/Sao_Paulo for granularity Some(DAY), got “2013-10-20T03:00:00Z” for start_time

If I change the start date by one day (“2013-10-19” or “2013-10-21”) then data is returned. In all three cases, I am specifying only the date, and not the time.

I used the following twurl calls to test this (the first one is the one that fails, the others are good):

twurl -H "/0/stats/accounts/519r8i/promoted_tweets?promoted_tweet_ids=1y34s,1yup1,C1z3d2,21f7t&start_time=2013-10-20&end_time=2013-11-19&granularity=DAY"
twurl -H "/0/stats/accounts/519r8i/promoted_tweets?promoted_tweet_ids=1y34s,1yup1,C1z3d2,21f7t&start_time=2013-10-19&end_time=2013-11-19&granularity=DAY"
twurl -H "/0/stats/accounts/519r8i/promoted_tweets?promoted_tweet_ids=1y34s,1yup1,C1z3d2,21f7t&start_time=2013-10-21&end_time=2013-11-19&granularity=DAY"

Why is this one date failing, when the dates around it work?