No need to adjust anything in the database - it's your format mask that's causing you funky time results. Format mask 'mm' (like 'MM') is for MONTHS, not for MINUTES as you used it. Minutes are denoted with 'MI' (or 'mi').

Use:

select to_char(sysdate, 'YYYY-MM-DD hh:MI:ss') from dual;

and everything will be back to normal...