Changes in date after importing från query in excel

Hi!

I've got an excel file with data (different measurements for water quality) from january 1980 to december 2018. 

I open the table in power query från excel to make quick changes in format in a few columns and deleting empty rows etc. When I save and apply the changes the dates in the table ain't the same as before. I can see that for example the dates starts with january 1984 and ends in year 2022... The new rows still has data in the rest of the columns. When I check a random other year, for example april 2014, the measurements ain't the same as in the original file for that date either. 

I know that the file, when it was first brought to use a number of years ago, was made in Mac-format for the dates, and now we use PC. I've heard that Mac and PC have different start-dates in their formats, but even if that is what's wrong I don't know how to fix it.

I have also added "19" in the date-column in the rows before year 2000 because the users had only been writing "801012" instead of "19801012". I don't know if adding 19 by merging two columns matters here. 

The pivoted table is about one million rows, so its quite big which gives me a little to none overview of the data which makes it even harder to see if there is a pattern to the changes. 

I've been struggling with this for many hours now and would be greatful for any guess in how to solve this!

/Mary