From the date of any Periodic Inspection and Test (PIAT), or 'hydro', the Standards state that a cylinder has to be re-hydro tested at the five-year anniversary of the previous one. The Standards also say that the cylinder should undergo a Periodic Inspection (PI) after two and a half years, to check it is fit to be used until the next PIAT.
If a cylinder misses a PI, or a 'visual' as most people know it, then common sense should prevail. If the cylinder is within a year to six months of the next Hydro then bring forward the hydro. When this situation happens, the five-year interval will commence from the date of the new hydro rather than the original one.