Poco handles DST as +3600s, however there exists also negative DST offset (-3600s).
Example is Europe/Dublin timezone, where poco erroneously returns offset 7200 instead of 0 during winter with recent tzdata/tzinfo package under Linux.

This line of code causes this behaviour:
https://github.com/pocoproject/poco/blob/poco-1.9.4-release/Foundation/src/Timezone_UNIX.cpp#L79

For background on this, see tzinfo/tzinfo#95 (comment)