Oracle TimeZones and Daylight savings time

Hi,
I am working on converting a bunch of old data to a new system. I ran a couple of validation reports for the end user to verify, it seems the times were off. I am in the EST but it seems the times in the db were stored as UTC.
So when you convert the time from UTC to EST you need to subtract 5 hours but when it is DST you only subtract 4.
I need to know how to figure out if a time was entered in DST or EST and then I can use something like a decode to decide how much to subtract.
DST Rules:
Stone Age -- 2006 First Sunday in April ,Last Sunday in October
2007 -- present DST Begins 2 a.m. (Second Sunday in March)      DST Ends 2 a.m. (First Sunday in November)
Here is part of my select statment:
to_date(to_char(asgn.start_date-5/24,'DD-MON-YYYY')) effective_from,
I looked at Ask Tom and other sites but could not find a good working example that didn't have a hard coded date. Maybe it is not possible in a select statement.
Thanks so much for any help you can provide !!!! :)

>
I need to know how to figure out if a time was entered in DST or EST
This seemed to do it:
>
I don't see how it possibly can. A DATE datatype does not contain timezone. See the definition of what it contains in the SQL Language doc
http://docs.oracle.com/cd/B28359_01/server.111/b28286/sql_elements001.htm#i54330
>
12
DATE
Valid date range from January 1, 4712 BC, to December 31, 9999 AD. The default format is determined explicitly by the NLS_DATE_FORMAT parameter or implicitly by the NLS_TERRITORY parameter. The size is fixed at 7 bytes. This datatype contains the datetime fields YEAR, MONTH, DAY, HOUR, MINUTE, and SECOND. It does not have fractional seconds or a time zone.
>
So that DATE value has NO way of knowing what the timezone of the process that entered it was.
And depending on the meaning of that DATE column you don't even know for sure if the date value represents a date in that process's timezone. I can enter a date representing '01-Jan-2007' but that date doesn't necessarily mean it belongs to my timezone.

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