Get records in ascending order of modified date

Get records in ascending order of modified date

Hi,

I am using below URL for getting Accounts records 

"/crm/v2.1/Accounts?sort_order=asc&sort_by=Modified_Time"

I am also setting a "If-Modified-Since" header to get records modified after the specified date. But I am getting older new modified date in first record and older modified record date in last record. Could you tell which sort order I need to set to get old modified date in first record and latest modified date in last record.

Thanks !