Hi Irfan,

Try to add the code in default value in the dictionary of the field you are using.

1) If Field type= Date

javascript:var glideGD=new GlideDateTime();glideGD.addDays(30);glideGD.getDate();

2) If Field type= Date/Time

javascript:var glideGDT=new GlideDateTime();glideGDT.addMonths(1);glideGDT.getDisplayValue();

https://developer.servicenow.com/dev.do#!/reference/api/madrid/server_legacy/c_GlideDateTimeAPI#r_GD...

Please mark as Correct and Helpful if your issue was resolved with the solution provided.

Regards,

Subhojit Das