How do I store and retrieve dates as utc to enable comparison?

I’m storing am expiry date as a UTC string using toISOString().

I want to check the user’s current datetime is less than the expiry date, but the user’s current datetime is in local time I think, using new Date().

The question is, do I just use new Date(expiryDatetime) (which converts it to local time? This is the part I’m not sure about)?

The above is what I’m trying, but I’m not sure if this will work correctly. I think it’d be easier if everything was in UTC but it does not appear to be trival to convert a datetime to UTC if it’s not a string.