WP timezone set as UTC does not work ok with Appointable products with Customer Timezones option enabled

[Valentina R]

To reproduce:

1. set WP timezone to UTC-5.
2. set up an appointable product with availability from 09:00 to 10:00 and the option customer timezones'.
3. open the product page and select the Timezone UTC-5 OR Chicago and you’ll get the slot at 08:00 - wrong.
4. Then disable the ‘custom timezones’ option and you’ll see the slot at 09:00 as expected.