A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
This is one way to do the changes if the dates are text and not recognized as dates at all (as the next post assumes). Try Sheeloo's method first and if it does not work, then try these formulas.
Assuming one of the dates is in cell A2, try this formula and format the cell as you desire:
=IF(ISERR(DATE(MID(A2,FIND(" ",A2)-4,4),LEFT(A2,FIND("/",A2)-1),MID(A2,FIND("/",A2)+1,2))),DATE(MID(A2,7,2),LEFT(A2,2),MID(A2,3,2)),DATE(MID(A2,FIND(" ",A2)-4,4),LEFT(A2,FIND("/",A2)-1),MID(A2,FIND("/",A2)+1,2)))
Now - if for those dates in the format 03.05.14 show up as 1914 instead of 2014, then change the formula to this:
=IF(ISERR(DATE(MID(A2,FIND(" ",A2)-4,4),LEFT(A2,FIND("/",A2)-1),MID(A2,FIND("/",A2)+1,2))),DATE(MID(A2,7,2)+100,LEFT(A2,2),MID(A2,3,2)),DATE(MID(A2,FIND(" ",A2)-4,4),LEFT(A2,FIND("/",A2)-1),MID(A2,FIND("/",A2)+1,2)))
This formula assumes that dates of the format 3/3/2014 have a space after the 2014 followed by the time of day as you have shown. It also assumes that the 03.05.14 type dates have 2-digit days and months in all cases.
There should be a date format available to provide you with the dd-mmm-yyyy format, but if there is not one, then use a custom format like that. That is the custom format entry would be:
dd-mmm-yyyy
After getting the dates converted you can select all of the cells with the formulas in them and use Edit Copy and then select all of the bad date entries and use Edit Paste Special with the Values option selected. That may cause the formulas to return errors or strange results, but it does not matter - after doing the Paste Special with Values, you may delete all of the formulas.