Hi Kotac,
Greetings from Syncfusion support.
You can directly set the date as string to the StartTime and
EndTime fields values like the below. We have prepared a sample for your
reference.
{
"Id": 1,
"Subject": "Story Time for Kids",
"StartTime": "2022-07-05T13:40:00+01:00",
"EndTime": "2022-07-05T16:00:00+01:00"
}
Sample: https://stackblitz.com/edit/react-e7gutz-qkdkkp?file=index.js,datasource.json
Output:

Kindly try the above sample and let us know if this meets
your requirement.
Regards,
Ruksar Moosa Sait