How to get a correct time zone info from a timestamp with no tz info

to_char((from_tz(cast(PERIOD_START_DTM as timestamp),'UTC') AT TIME ZONE 'Europe/Berlin'),  'YYYY-MM-DD HH24:MI:SS TZH:TZM')

Format of time and date in Oracle

Parameter Explanation      
YEAR Year, spelled out      
YYYY 4-digit year      
YYY        
YY        
Y Last 3, 2, or 1 digit(s) of year.      
IYY        
IY        
I Last 3, 2, or 1 digit(s) of ISO year.      
IYYY 4-digit year based on the ISO standard      
RRRR Accepts a 2-digit year and returns a 4-digit year.      
A value between 0-49 will return a 20xx year.      
A value between 50-99 will return a 19xx year.      
Q Quarter of year (1, 2, 3, 4; JAN-MAR = 1).      
MM Month (01-12; JAN = 01).      
MON Abbreviated name of month.      
MONTH Name of month, padded with blanks to length of 9 characters.      
RM Roman numeral month (I-XII; JAN = I).      
WW Week of year (1-53) where week 1 starts on the first day of the year and   continues to the seventh day of the year.
W Week of month (1-5) where week 1 starts on the first day of the month and ends   on the seventh.
IW Week of year (1-52 or 1-53) based on the ISO standard.      
D Day of week (1-7).      
DAY Name of day.      
DD Day of month (1-31).      
DDD Day of year (1-366).      
DY Abbreviated name of day.      
J Julian day; the number of days since January 1, 4712 BC.      
HH Hour of day (1-12).      
HH12 Hour of day (1-12).      
HH24 Hour of day (0-23).      
MI Minute (0-59).      
SS Second (0-59).      
SSSSS Seconds past midnight (0-86399).      
AM , A.M., PM, or P.M. Meridian indicator      
AD or A.D AD indicator      
BC or B.C. BC indicator      
TZD Daylight savings information. For example, ‘PST’      
TZH Time zone hour.      
TZM Time zone minute.      
TZR Time zone region.