Excel Date Format Issue

Registration date
Monday July 6, 2015
Last seen
July 6, 2015
Regardless of how I format a cell containing a date, it inserts an equal sign and displays 01/00/1900. I've changed the date format different ways, but it doesn't help. E.G. I type 3/24/1953 and it changes it to =3/24/1953, and displays 01/00/1900. The only thing I've been able to do is use the date function to get it to display correctly. But I can't keep doing that (too many dates). There's a way to fix this, but I don't recall what it is. Does anyone know? Oh yes...this spreadsheet came from a guy who probably converted it from Lotus. His dates show up properly until I try to change a date...then it goes to 01/00/1900. It's not just a display issue. The calculations are actually using 01/00/1900 to calculate an age.