The minimum notice period:
Begins on the day the notice is received by the tenant and ends on the day falling 3 months after began or whatever longer period after it began as the landlord and tenant have agreed between them.
The reference to a period of 3 months is to a period which ends in the month which falls 3 months after the month in which it began, either:
On the same day of the month as it began or
If the month in which the period ends has no such day, on the final day of the month