Some dates recognized as dates, some dates not recognized. Why?

Question

Here is a list of dates:

04-22-11
12-19-11
11-04-11
12-08-11
09-27-11
09-27-11
04-01-11

When you copy this list in Excel, some of th

Answer 1

This is caused by the regional settings of your computer.

When you paste data into excel it is only a bunch of strings (not dates).

Excel has some logic in it to recognize your current data formats as well as a few similar date formats or obvious date formats where it can assume it is a date. When it is able to match your pasted in data to a valid date then it will format it as a date in the cell it is in.

Your specific example is due to your list of dates is formatted as "m/d/yy" which is US format. it pastes correctly in my excel because I have my regional setting set to "US English" (even though I'm Canadian :) )

If you system is set to Canadian English/French format then it will expect "d/m/yy" format and not recognize any date where the month is > 13.

The best way to import data, that contains dates, into excel is to copy it in this format.

2011-04-22
2011-12-19
2011-11-04
2011-12-08
2011-09-27
2011-09-27
2011-04-01

Which is "yyyy-MM-dd", this format is recognized the same way on every computer I have ever seen (is often refered to as ODBC format or Standard format) where the units are always from greatest to least weight ("yyyy-MM-dd HH:mm:ss.fff") another side effect is it will sort correctly as a string.

To avoid swaping your regional settings back and forth you may consider writting a macro in excel to paste the data in. a simple popup format and some basic logic to reformat the dates would not be too difficult.