FreeTDS and dates

Explanation of how pymssql and FreeTDS can break dates.


Make sure that FreeTDS is compiled with --enable-msdblib configure option, or your queries will return wrong dates – "2010-00-01" instead of "2010-01-01".


There’s an obscure problem on Linux/*nix that results in dates shifted back by 1 month. This behaviour is caused by different dbdatecrack() prototypes in Sybase Open Client DB-Library/C and the Microsoft SQL DB Library for C. The first one returns month as 0..11 whereas the second gives month as 1..12. See this FreeTDS mailing list post, Microsoft manual for dbdatecrack(), and Sybase manual for dbdatecrack() for details.

FreeTDS, which is used on Linux/*nix to connect to Sybase and MS SQL servers, tries to imitate both modes:

  • Default behaviour, when compiled without --enable-msdblib, gives dbdatecrack() which is Sybase-compatible,
  • When configured with --enable-msdblib, the dbdatecrack() function is compatible with MS SQL specs.

pymssql requires MS SQL mode, evidently. Unfortunately at runtime we can’t reliably detect which mode FreeTDS was compiled in (as of FreeTDS 0.63). Thus at runtime it may turn out that dates are not correct. If there was a way to detect the setting, pymssql would be able to correct dates on the fly.

If you can do nothing about FreeTDS, there’s a workaround. You can redesign your queries to return string instead of bare date:

SELECT datecolumn FROM tablename

can be rewritten into:

SELECT CONVERT(CHAR(10),datecolumn,120) AS datecolumn FROM tablename

This way SQL will send you string representing the date instead of binary date in datetime or smalldatetime format, which has to be processed by FreeTDS and pymssql.