To be exact (you really have to be with the CRO), it's
1. 9 months and 28 days for a paper return (B1 & accounts)
2. 9 months and 56 days for an electronic return (but you have to file the B1 online after 9 months & 28 days, thereby getting an extra 28 days to file the accounts)
Also if you are filing a paper return and your return date falls on a weekend day then you get until the next working day to file the documents but with an electronic return, you DON'T get until the next working day, it must be filed before the exact deadline day (last year I had to file one on Christmas Day!)