The board of water supply may fix and adjust rates and charges for the furnishing of water and for water services so that the revenues derived therefrom shall be sufficient to make the waterworks and water systems self-supporting and to meet all expenditures authorized by this part; the board may establish variable rates among the several districts of the county, or among the areas served by the individual water systems within the county, for the purpose of establishing charges as closely as possible to the necessary amount required for the maintenance and operation of the particular individual water systems; provided no rates and charges shall be fixed or adjusted prior to the holding by the board of a public hearing, public notice of which shall have been given not less than twenty days before the date set for the hearing. The notice shall state the time and place for the hearing and the proposed rates and charges to be considered thereat. The time within which the notice shall be given shall be computed by including the first day (the day of notice) and excluding the last day.