How to add Addmonth() in zoho crm using client script

How to add Addmonth() in zoho crm using client script

Hello Community,

I have one requirement in Zoho CRM. There is Plan duration dropdown field. For ex, I have selected 1 month plan, when I entering date in Plan Start Date, Plan End Date field should automatically filled with Plan Start Date + addMonth(1). With Zoho deluge, we can easily bring it on. But with client script I have tried, it shows as like this as per below screenshot. The date exceeds more than 31. How to handle this? If anyone knows, please suggest me. 

Thank you!