subtract and add time having both negative and positive time in javascript

Question

-12:00 - 5:30 => -6:30

-2:00 - 5:30 => 3:30

00:00 - 5:30 => -5:30

6:00 - 2:30 => 3:30

i want to subtract time having both positive and negative index

let myCountries = [

    {
        countryName: "NewZealand",
        timeDiff: "-06:30",
        id: 5,
        timeZone: "Pacific/Auckland"
        
    },
    { 
        countryName: "China",
        timeDiff: "-02:30",
        id: 3,
        timeZone: "China Standard Time"
    },
    
    { 
        countryName: "GMT",
        timeDiff: "+05:30",
        id: 4,
        timeZone: "GMT"
    },
    { 
        countryName: "India",
        timeDiff: "+00:00",
        id: 6,
        timeZone: "Asia/Kolkata"
     },
     { 
        countryName: "Italy",
        timeDiff: "+03:30",
        id: 2,
        timeZone: "CET"
    }
    
]

a = "12:00";
b = "-5:30";

let beta = parseInt(a.split(":")[0]);
let gamma = parseInt(b.split(":")[1]);

Now here i want to update timeDiff with respect to the result of beta - gamma which is 6:30

So i have to now add 6 hours and 30 minutes from all the objects

After performance operation the myCountries will be as below :-

[

    {
        countryName: "NewZealand",
        timeDiff: "00:00",
        id: 5,
        timeZone: "Pacific/Auckland"
        
    },
    { 
        countryName: "China",
        timeDiff: "04:00",
        id: 3,
        timeZone: "China Standard Time"
    },
    
    { 
        countryName: "GMT",
        timeDiff: "+12:00",
        id: 4,
        timeZone: "GMT"
    },
    { 
        countryName: "India",
        timeDiff: "06:30",
        id: 6,
        timeZone: "Asia/Kolkata"
     },
     { 
        countryName: "Italy",
        timeDiff: "+10:00",
        id: 2,
        timeZone: "CET"
    }

a = "-12:00"; b = "=5:30";

let beta = parseInt(a.split(":")[0]);

let gamma = parseInt(b.split(":")[1]);

If the result of will be -06:30

Now i have to subtract result from timeDiff

i have already tried momentjs

Answer 1

checkout this

function diff(start, end) {
    start = start.split(":");
    end = end.split(":");
    var startDate = new Date(0, 0, 0, start[0], start[1], 0);
    var endDate = new Date(0, 0, 0, end[0], end[1], 0);
    var diff = endDate.getTime() - startDate.getTime();
    var hours = Math.floor(diff / 1000 / 60 / 60);
    diff -= hours * 1000 * 60 * 60;
    var minutes = Math.floor(diff / 1000 / 60);
    
    return (hours < 9 ? "0" : "") + hours + ":" + (minutes < 9 ? "0" : "") + minutes;
}