For any API calls that contain a date field (e.g. date_from and date_to in the Access report call) this field should be in the format of a UNIX timestamp.
You can use https://www.unixtimestamp.com/ to convert the date to a UNIX timestamp (in UTC time).
Our recommendation here would be to generate the timestamps programmatically using DateTime objects with proper timezones in the language of your choice, as UTC does not have daylight savings, so whatever results you get from manually adding/subtracting hours will not be valid at certain points in the year.