Get number of days after subtracting 2 dates which is in ISO String format

Question

I want to calculate the number of days after the difference between 2 dates (Both dates in isoString format). When I try to subtract it returns NaN instead of the days. Here is my code, may I know where I went wrong?

my HTML

<ion-datetime displayFormat="DD-MM-YYYY" done-text="Select" min="2020" max="2021" placeholder="Forward Date" (ionChange)="selectDate($event)"></ion-datetime>

my .ts file

 selectDate(value){
    this.nextDate= new Date(+new Date() + 86400000+ 86400000).toISOString();  // date after 2 days from current date
    this.check=value.detail.value;
    console.log(this.check) // 2020-06-29T12:01:57.100+05:30 (This date is obtained from ion dateTime picker which is already in toISOString format)

    console.log(this.nextDate) // 2020-06-25T07:02:22.513Z
    this.ihe=this.check-this.nextDate; // Gives Nan
    console.log(Math.round(this.ihe/ (1000 * 3600 * 24)))
  }

Here nextDate is predefined date and check variable contains date selected from date picker.

Answer 1

You try to calculate two strings. That's why you get a NaN. You should transform the two dates to integers. For example:

selectDate(value){
    this.nextDate= new Date(+new Date() + 86400000+ 86400000).toISOString();  // date after 2 days from current date
    this.check=value.detail.value;
    console.log(this.check) // 2020-06-29T12:01:57.100+05:30 (This date is obtained from ion dateTime picker which is already in toISOString format)

    console.log(this.nextDate) // 2020-06-25T07:02:22.513Z
    this.ihe=new Date(this.check).getTime()-new Date(this.nextDate).getTime(); // Gives Nan
    console.log(Math.round(this.ihe/ (1000 * 3600 * 24)))
}

In this example, you transform the date string on the fly to a Date Object on which you can use getTime(). By that, you get the delay between 1.1.1970 and the Date in milliseconds.