Usually dates for meetings are set well in advance, often annually, and the Chair carries no greater authority than anyone else with regard to those dates/times. However, meetings do sometimes have to be changed if it is unlikely they will be quorate or the venue is unavailable. It is usual then for the clerk to be asked to make alternative arrangements. Changing dates and times is to be discouraged as it confuses the public who might attend, as well as members, but it does happen from time to time for perfectly innocent reasons.