How do you pass a time range to a stats API?

The SmartDocs page for the "Retrieve and Filter Metrics for a Dimension", says to pass a timeRange as:

MM/DD/YYYY HH:MM~MM/DD/YYYY HH:MM. 

For example:

8/24/2015 1200:00~9/20/2015 1200:00 

That works fine in SmartDocs because it URL encodes the time range in the generated curl command:

curl -X GET "https://api.enterprise.apigee.com/v1/organizations/sgilson/environments/test/stats/apis?select=sum(message_count)&timeRange=8%2F24%2F2015%201200%3A00~9%2F20%2F2015%201200%3A00&timeUnit=day" 

But in a straight curl command, how do you specify the time range? The doc says specify the date as MM/DD/YYYY%HH:MM using a % to precede HH:MM, as in:

curl  https://api.enterprise.apigee.com/v1/o/{org_name}/environments/test/stats/target_url?"select=sum(is_error)&timeRange=8/08/2015%1200:00~9/15/2015%1200:00&timeUnit=week" 

But that generates an error:

{
  "code" : "Invalid parameter - timeRange",
  "message" : "Invalid parameter - timeRange",
  "contexts" : [ ]
} 

If I do it like SmartDocs, by omitting the %, then I get no response at all:

curl  https://api.enterprise.apigee.com/v1/o/{org_name}/environments/test/stats/target_url?"select=sum(is_error)&timeRange=8/08/2015 1200:00~9/15/2015 1200:00&timeUnit=week" 

How do I pass the range then in curl?

Thanks,

Stephen