So you are asking then, removing the thought that the new-calendar ever even happened - is there anything about it that is heretical by itself.
Heres my answer:
1) Holy Tradition has always demanded that we fast like the Apostles after Pentecost. The new-calendar s a teaching instrument of the Church, on certain years has this fast completley removed, which is a teaching against the Church.
2) The Churches calendar is not independent. It works hand-in-hand with the Typicon and the Typicon's formula's work this year, next year, and forever! But with the new-calendar, there are combinations of schedules and services which have to be made up on the fly! In this regard, it makes the service books, which are a part of Holy Tradition, obsolete.
3) A piece of wood is a neutral object. It has no religious value whatsoever. It could be cast into the fire or chopped up. It makes no difference. But as soon as an iconographer writes the words of the Lord on it, it becomes an Icon and is Holy - it has no need of being blessed. It cannot be chopped up or defaced in any way.
Likewise the calendar is a neutral object until the Church imprints God's holy days on it. It then forever becomes an icon of time. The new-calendar casts this icon of time, which took hundreds of years to develop, and it into the fire; the new-calendar declares the festal order of the Church Fathers to be in error; it overturns the festal relation between the Paschal cycle and immovable feasts; it abolishes fasts; it changes immovable feasts to movable ones (for example, the feast of Saint
George); But most of all, it destroys the work of the Holy Spirit, who does not make mistakes, and who guides the Orthodox in all truths!
Is the use of the new-calendar a heresy, without any other consideration? I would only ask why is it important to know?