What is your definition of 'unit of time'? Neither 'month' nor 'day' have fixed durations (the latter due to leap seconds). So why should we consider 'day' a un

What is your definition of 'unit of time'? Neither 'month' nor 'day' have fixed durations (the latter due to leap seconds). So why should we consider 'day' a unit of time but not 'month'?