As a freelancer, a company sends me some spreadsheets.  I copy the data and
paste it into a new spreadsheet (to preserve the integrity of the original
data) then manipulate as appropriate.  

However, the date field is not copying and pasting correctly.  The original
spreadsheet is some sort of txt file (downloaded from an archaic database
system - I am trying to get them to send me a copy of this orginal txt file)
which they convert to an excel spreadsheet - all on a PC. Fine.  They zip it,
send it to me.  I am on Mac.  I unzip and look at the information.  The date
field is correct.  When I copy the date field and paste it into a new
document, it changes the number date (adds one day) and the year (adds 4
years).  

Has anyone else experienced this?  Anyone have any ideas for
troubleshooting?  I double checked the cell format (as number type = date)
for the original spreadsheet and on the new spreadsheet.  

Thanks so much!