Why the timestamps in the database are different from the timestamps in ServiceNow?
ServiceNow stores all the date and time related fields in the UTC timezone. What you see in the ServiceNow user interface is just a display setting of your user account, timezone configuration. SnowMirror stores the timestamps in a way it receives them from ServiceNow. So all timestamps are stored in the UTC timezone. You have to convert those values into your timezone if you need it for reporting purposes.
Why it is not a good idea to store the dates in your own timezone? What if you have users in several timezones? How about the DST?