I want to figure out how many days have passed from the start date. I'm using the function =TODAY()-C3 for that. The start date is located in C3, as seen in the image. Yet, the response will arrive "03-Jan-00." That works if I only type =TODAY() or =TODAY()+5. For your benefit, I have attached a snapshot and a Google Drive link where you may view a screen capture of the issue.
https://drive.google.com/file/d/1lQ7djK1BRICjFfAaAG0T08snuRZhcU-k/view?usp=share_link