What does a timespan within the events endpoint refer to?
The attribute object within when refers to either timespan which is A span of time with a specific beginning and end time or day (which is an all day event). Please find more details regarding all the objects and fields for an event here >> Event API references.
The end_time and the start_time on the example above are in the UNIX timestamp, there are multiple ways to convert this to normal DateTime, for example in python this would look as below while converting to the UTC timezone.
import datetime
dt_utc = datetime.datetime.utcfromtimestamp(1650763800)
print(dt_utc)