A date/time vector is defined as daily if the vector has not more
than one date/time stamp per day.
A date/time vector is defined as monthly if the vector has not more
than one date/time stamp per month.
A date/time vector is defined as quarterly if the vector has not more
than one date/time stamp per quarter.
A monthly date/time vector is also a daily vector, a quarterly date/time
vector is also a monthly vector.
A regular date/time vector is either a monthly or a quarterly vector.
NOT yet implemented is the case of weekly vectors.