convert current time to format along with timezone

[#1]

how can i convert the system time to format as follows

YYYYMMDDhh:mm:ss-zz:zz

If its eastern the format is 2015091015:10:15-04:00.

Tried to use the following from another question

var timestamp = context.getVariable("system.timestamp"); 

var mydate = new Date(timestamp); 

var formatted_date = mydate.toString('yyyyMMddhh:mm:ss');

But it keeps on printing in the following format

formatted_date ThuMay28201500:19:10 GMT-0000(UTC)