Numbers converting to dates when opening worksheets

[BudHewitt]

I'm having a problem similar to others posted here but none of the solutions tell me or work to recover a whole workbook. I have been using the workbook for years with many pages and all of a sudden when I opened it all the 'not-otherwise-specifically-formatted' cells display as dates. I have several different cell formats re text size, alignment, bold etc. but changing to a new style changed all on the one page I tried to a single format albeit with the correct "general" number format. I considered copying the data to a new sheet in the workbook but the new sheet is already default formatted as a custom date format. Does anyone know how to recover my workbook without painstakingly going cell by cell or row by row?

My problem got worse today after correcting and reformatting one sheet yesterday it reverted to dates again when opened today. I have passed a deadline to print out many pages and the problem is getting worse. Since this worrbook worked fine for literally years it must be something I did inadvertantly. While I would like to know what caused the problem so I don't do it again my most immediate problem is recovering my data correctly.

[BudHewitt]

I stumbled on a fix! Knowing someone on the forum spoke about a "-409" entry I noticed when trying to repair cell by cell there was a "-409" entry in the box that opened with the custom number type highlighted. On a whim I deleted it and instantaneously the formatting in the whole spreadsheet was corrected. This is the complete line I deleted:
[$-409]d-mmm-yy;@
My revised questions now are: How did I put that there? How do I avoid it in the future? and Can you explain what the line means? What is the connection with this line in this place with globally changing the number format in the whole workbook?