How do you check if a Custom Field in a YouTrack Workflows script is a Date?

If I access issue.fields['my-field'], it turns out to be a simple timestamp as a number both for a Date and a DateTime. How do I test if my-field is a Date and not a number?

I tried accessing its fields like dateTimeType and dateType, but since it’s just a number, I got undefined for both. Simply guessing if it’s a date based on the number’s value is also not gonna work in my case, because I need to be 100% sure.