If you are fortnightly paid you can have 27 fortnights (54 Weeks), rather than the normal 26. Revenues computer systems cannot handel this, or 14 4 weekly (56 Weeks) periods etc. Only 53 Weeks, which is indicated by a flag on the P35 return and your P60. So you will always have to watchout if they recalculate your tax (PAYE) for the year. Their system will use 52 weeks or 53 if the 'Week 53' flag is set. They also cannot handle multiple employments in the same year with the same employer. A 'bug' they told we they were going to fix when they introduced their new Year 2000 complient computer system...