Ok I realize this is a confusing one, but the way you have to call is like this:
twurl -t -H ads-api.twitter.com '/1/stats/accounts/18ce53x6d8b?entity=ACCOUNT&start_time=2016-05-13T00:00:00-03:00Z&end_time=2016-05-15T00:00:00-04:00Z&granularity=DAY&metric_groups=BILLING&placement=ALL_ON_TWITTER'
The end time in v1 is exclusive, meaning it will include up until that minute but not including it. Because the time zone of that minute is already a different time zone definition, you have to pass it the new time zone offset (I’m not sure if this makes sense or not, will try to discuss it internally and compare vs old behavior).
Then after that 15th, you can call with the correct UTC-4 on both start and end:
twurl -t -H ads-api.twitter.com '/1/stats/accounts/18ce53x6d8b?entity=ACCOUNT&start_time=2016-05-20T00:00:00-04:00Z&end_time=2016-05-26T00:00:00-04:00Z&granularity=DAY&metric_groups=BILLING&placement=ALL_ON_TWITTER'