How to adjust Date plus and minus,since the date is not same for all countries

[gctuser]

How to adjust Date plus and minus, the date is not same for all countries
For example in Oman Current Hijri Date is
Georgian Date: 19/06/2019
Hijri Date:15 Syawwal1440

but https://zulns.github.io/Calendar.js/index.html
showing 16 syawwal, we want to minus 1, please let me know how to do so.

[Mohsan469]

I want to add 1 day for Saudi Arabia. Please update the solution if anyone have.