@Anil74, You can use gs.monthsAgo(15) function to get a 15 months before today's date. Please refer below SS for syntax :
You can use below code to get 15 months date before a given date (not today's date) :
var dateA = "2023-02-28 08:00:00";
var gdt = new GlideDateTime(dateA);
gdt.addMonths(-15);
gs.info(gdt.getDate());
Please mark helpful and accept solution if it helped you.
