Hello,
There is an easy and a hard way to implement this behavior. The easy way is using the Date/Time module distributed with the Developer and Platinum versions of the plugin, the hard way is using directly the Date object of javascript.
For example, assuming that the date/time field is the fieldname1.
The easy way:
GETDATETIMESTRING(DATETIMESUM(fieldname1, 'mm/dd/yyyy', 4, 'm'), 'mm/dd/yyyy')
The hard way:
(function(){
var d = new Date(fieldname1*86400000);
d.setMonths(d.getMonths()+4);
return CDATE(d/86400000,'mm/dd/yyyy');
})()
The support service does not cover the implementation of the users’ projects (forms or formulas), but if you need additional help implementing your project, I can offer you a custom coding service from my private website:
https://cff.dwbooster.com/customization
Best regards.