May I try to suggest a rather differing point of view to the majority, Su51nan?
Reading your post, I gathered, perhaps incorrectly, that you gave up a paid position two years ago, in order to work with your husband in his business, or a joint venture, you and he started, renovating properties full-time.
In other words, you made a commitment to your husband, which forms the basis for your and his livlihood at the present time.
Do you honestly feel you are being fair to him, turning round now and saying that you only want to work four days a week in your joint business, as you intend to spend on week-day looking after your grandchild?
You do not say that your daughter will be paying you for child-care, ( I gather she is not intending to do so) so you are basically depriving yourself and your husband of a day's earnings.
He has made it clear, whether nicely or not, I do not know, that he is afraid this will affect your and his work, which it obviously will.
I realise you have offered to work all either Saturday or Sunday - but hang on a second. Can you cope with working six days a week instead of five, and is it fair to either have less time off with your husband, or to off-load some of the household tasks you usually both do at the weekends onto him?
Admittedly, I too would have resented a remark that spending more than a couple of hours with a grandchild was making her the centre of my life, but was this remark made in the course of a heated discussion, or has your husband always resented the time you spend with our children?
Frankly, right now it sounds to me as if you have, in the joy of being a new grandmother, not thought this through properly, and resent your husband's point of view.
So could you both try to reconsider your positions on this?
And have you thought what might happen if your daughter suddenly decides she needs more than one day's help a week from you?