Formatting Birthdates Issue

I have been having some trouble with getting some birthdates formatted the way I want. They are part of a report imported from my company's system in a .txt file that I converted to .xlsx. When I went through the conversion process, a majority of the dates formatted properly (m/d/yyyy) but there are a couple thousand that stayed as just a number with zero punctuation (/). This has happened on a few reports with older birthdates before the millennial generation, but this one and one more have thousands with this issue. And I would rather not have to go through and manually input / / into thousands of dates. 

These dates on this particular report range from 1/13/1949 to 9/30/1964, or as shown on my file 1131949 to 9301964.

There are thousands of other cells in the column that took to the formatting properly and show the proper dates. 

I have tried to do a few different things to fix this but to no avail.

1. Changing the format from General to Short Date changes the date
   • 1131949 turns into 3/2/4999
2. Highlighting the column and selecting Text to Columns and setting the Column data format to Date and MDY
   • has no change and results in the same date change when converting to a short date.
3. File>Options>Advanced>When Calculating this Workbook:
   • Selected the 1904 date system box
     • made it convert to a further date in the future
4. Added a leading zero to all the affected cells to prevent Excel from thinking it was serial
   1. Still converts to 3/2/4999 when selecting the short date option.
5. Changing the format to number from general then switching to short date.
6. Doing a custom date format in the format cells menu.

Can someone please tell me what it is I need to do to fix this problem, otherwise I'll be spending hours just typing a stupid / over and over.