You must inherit class pos_details() in your module and add order='date_start asc' clause, like this:
class pos_details(osv.osv_memory):
_inherit = 'pos.details'
def print_report(self, cr, uid, ids, context=None):
if context is None:
context = {}
datas = {'ids': context.get('active_ids', [])}
res = self.read(cr, uid, ids, ['date_start', 'date_end', 'user_ids'], order='date_start asc', context=context)
# or # res = self.read(cr, uid, ids, ['date_start', 'date_end', 'user_ids'], order='date_end asc', context=context)
res = res and res[0] or {}
datas['form'] = res
if res.get('id',False):
datas['ids']=[res['id']]
return self.pool['report'].get_action(cr, uid, [], 'point_of_sale.report_detailsofsales', data=datas, context=context)