The example date/time you provided can be translated as January 29, 2021 8:00 AM (plus seven seconds, Central Standard Time). The date/time format used by the ActiveCampaign API is ISO 8601 format. The format looks like this:
YYYY-MM-DDThh:mm:ss+TZ
Where each component is:
YYYY = four-digit year
MM = two-digit month (01=January, etc.)
DD = two-digit day of month (01 through 31)
hh = two digit hour (00 through 23)
mm = two digit minute (00 through 59)
ss = two digit second (00 through 59)
TZ = time zone designator (+hh:mm or -hh:mm)