Problems with date calculations in sheet

Hi. I wonder if someone can help me. I'm a volunteer book review editor. I use Zoho Sheet to keep track of books out. I have a column where I keep the date a reviewer receives a book, and another column that tracks when the review is received. I check that the date reviewer receives the book not empty, and that the date the review is received is empty. The reviewer has the book, but has not submitted a review. In another column, I'm wanting to compute the date the review is due -- 30 days from when the book is received by the reviewer. I've tried the following formula in the Date Review Due field -- =IF(AND( K2 <> "" ; N2 = "" );NOW()- K2 ; "" )

The result is a date in 1900.l

I've attempted to attach a screen shot. If anyone can help, I'll be very grateful.

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