case 'float_time':
            var pattern = '%02d:%02d';
            if (value < 0) {
                value = Math.abs(value);
                pattern = '-' + pattern;
            }
            var hour = Math.floor(value);
            var min = Math.round((value % 1) * 60);
            if (min == 60){
                min = 0;
                hour = hour + 1;
            }
            return _.str.sprintf(pattern, hour, min);

In openerp 7 custom module on_change function used based on difference between two time (float). Actual requirement if the output negative value not shows in the text box user can manually enter the value or change the format 00:00 into 0:0

def onchange_time(self, cr, uid, ids, start_time, end_time, context=None):
          if diff_time < 0: 
               diff_time = 0.0
               # how to call js method _.str.sprintf(pattern, hour, min);   
               # and change the  pattern = '%01d:%01d'; or shows output None for Negative value only
          v['diff'] = diff_time
 return {'value': v}