Javascript parsing json date timezone return one day earlier in British Standard Time -

i have used below code date.

function parsejsondate(jsondate, dateformat) {      var offset = new date().gettimezoneoffset() * 60000;     var parts = /\/date\((-?\d+)([+-]\d{2})?(\d{2})?.*/.exec(jsondate);     if (typeof dateformat == 'undefined') {         dateformat = 'mm/dd/yyyy h:mm';     }     if (parts[2] == undefined)         parts[2] = 0;      if (parts[3] == undefined)         parts[3] = 0;      return new date(+parts[1] + offset + parts[2] * 3600000 + parts[3] * 60000).format(dateformat); }; 

but function returns 1 day earlier in british standard time. while input json date in correct. can me fix issue.