I used Text Wrangler to look at the original file and the Excel-saved file that I made. It is all gibberish to me but the two files were very different.
An Applescript might be useful for someone who doesn't have Excel around to fix the dates. There is probably a better way to do it that what I am going to post here but this is one way to do it. For all selected cells, it will first test that it contains a number then it will enter the conversion formula (except for the correction for the first 59 days of 1900), then it will replace the formula with the result.
tell application "Numbers" to tell the front document to tell active sheet to tell (first table whose class of selection range is range)
tell the selection range to repeat with i from 1 to count cells
tell cell i
try
if its value = (its value) / 1 then
set its value to "=\"1/1/1900\"+" & (its value) & " - 2"
set its value to (its value)
end if
end try
end tell
end repeat
end tell
And I just found another problem with this forum. I am unable to copy/paste a script's text from Applescript Editor into the forum, it justifies it off to the left side of the window. I had to "paste and match style". below is the result of copy/paste
tell application "Numbers" to tell the front document to tell active sheet to tell (first table whose class of selection range is range)
tell the selection range to repeat with i from 1 to count cells
tell cell i
try
if its value = (its value) / 1 then
set its value to "=\"1/1/1900\"+" & (its value) & " - 2"
set its value to (its value)
end if
end try
end tell
end repeat
end tell
