How to save date/time in an event using the Events API (stored as string instead of date)

Hello everyone,
I hope you’re all doing well.

I have a question about sending date/time values via the Klaviyo Events API.

I’m trying to store a date-time value in an event property, but it is always saved as a string instead of a date in Klaviyo. I’ve tried several formats, but the result is the same.

For example, I want the following value to be stored as a date, not a string:

expiry_date: "2022-11-08T00:00:00+00:00"

Below is a sample of the event payload I’m sending:

{ "CUS-stockist-name": "OLD-STKST",

"Coupon": {

      "minimum_order_amount": 0,

      "expiry_date": "2022-11-08T00:00:00+00:00",

      "amount": 500,

      "code": "BDAY-1SEDQNB"

}, "recommended-stockists": [ { "distance_km": 0.11, "address": "ADDR3F", "id": 3259, "url": "https://xxxxxxx.jp/stores/3259/", "name": "TEST-STK" } ], "CUS-stockist-id": 4667, "group-stockist": false, "$value_currency": "JPY", "$value": 500 }

Is there a specific way or required field type to ensure Klaviyo recognizes this as a date/time property instead of a string?
Or is it expected behavior that event properties are always stored as strings?

Any guidance or best practices would be greatly appreciated.
Thank you in advance!