How can I set my reactive form date input value?

Try the built-in formatDate package.

import { formatDate } from '@angular/common' 

and then convert your date and set it to the datepicker control

this.yourForm.controls.controlName.setValue(formatDate(date,'yyyy-MM-dd','en'));

You have two possibilities to set you date as default value.

Variant 1:

For setting default date value to <input> tag with type date you should use HTML-property value of input-tag.

Example:

<form [formGroup]="editForm2">
    <input type="date" formControlName="startDate" value="{{ post.startDate | date:'yyyy-MM-dd' }}">
    <input type="date" formControlName="endDate" value="{{ post.endDate | date:'yyyy-MM-dd' }}">
</form>

Variant 2 (recommended):

You can use an built-in function formatDate() in angular.

Step 1:

Import formatDate() from @angular/common in you component typescript file.

import { formatDate } from '@angular/common';

Note: formatDate(yourDate, format, locale) expects 3-4 parameters.

Step 2:

Format your dates in definition of your form group.

this.editForm = this.formBuilder.group({
      startDate: [formatDate(this.post.startDate, 'yyyy-MM-dd', 'en'), [Validators.required]],
      endDate: [formatDate(this.post.endDate, 'yyyy-MM-dd', 'en'), [Validators.required]]
});

Here is working example: https://stackblitz.com/edit/angular-kucypd

Here is documentation of input type date: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/date