Section 6C. In all actions based on contractual obligations, upon a verdict, finding or order for judgment for pecuniary damages, interest shall be added by the clerk of the court to the amount of damages, at the contract rate, if established, or at the rate of twelve per cent per annum from the date of the breach or demand. If the date of the breach or demand is not established, interest shall be added by the clerk of the court, at such contractual rate, or at the rate of twelve per cent per annum from the date of the commencement of the action, provided, however, that in all actions based on contractual obligations, upon a verdict, finding or order for judgment against the commonwealth for pecuniary damages, interest shall be added by the clerk of the court to the amount of damages, at the contract rate, if established, or at a rate calculated pursuant to the provisions of section six I from the date of the breach or demand. If the date of the breach or demand is not established, such interest shall be added by the clerk of the court from the date of the commencement of the action.