Provision of Length of notice i.e. 21 clear days is governed by section 171.
If you SGM is on January 31, 2010 than last date for dispatch of SGM notice would be January 07, 2010. It means notice is required to be posted around25 days in advance.
21 days (section 171) + 1 day for dispatch (court precedent) excluded + 1 days of SGM excluded (court precedent) + 2 days/48 hours in transit excluded (Section 53) === A total of 25 days.
This is my personal understanding. This calculation remains same for any kind of GM.