microsoft excel – How to use the value of a cell to write a formula in the same cell?

I have a spreadsheet here. On Cell J2, I need to write a formula, but the problem is that I can’t overwrite the cell as it contains the value which are to be needed for the formula. I can’t add a new column as it is the requirement. I need to write the following formula on J2 using the current value of J2.
=IF(ISBLANK(H2),"((TODAY()-(F2))*24)-(G2)",J2)

The left columns are intentionally kept blank as I am not supposed to share those information. However those are not necessary for the calculations.

How I am supposed to do it?enter image description here