How to create an extension method in TypeScript for ‘Date’ data type

You need to change the prototype:

interface Date {
    ConvertToDateFromTS(msg: string): Date;
}

Date.prototype.ConvertToDateFromTS = function(msg: string): Date {
    // implement logic
}

let oldDate = new Date();
let newDate = oldDate.ConvertToDateFromTS(TS_VALUE);

Though it looks like you want to have a static factory method on the Date object, in which case you better do something like:

interface DateConstructor {
    ConvertToDateFromTS(msg: string): Date;
}

Date.ConvertToDateFromTS = function(msg: string): Date {
    // implement logic
}

let newDate = Date.ConvertToDateFromTS(TS_VALUE);

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