Cell contents beyond 65536 vanish when opening xls file from Calc

Hello, I need some guidance on a problem that I have with my Calc tool. I am running LibreOffice v4.2.3.3 Engish ver.
on a Windows 7 (English ver.) 64-bit Dell Latitude E5530 machine that has 8GB of RAM and a Intel i7@2.9GHz.
I load a file with numerous data in a Calc spreadsheet that requires about 170,000 rows and 16 columns.
Data loads with no problem, I can process them by adding four extra columns on the same spreadsheet,
such as median, median of absolute deviation, etc, I save the sheet and close it. When I open it again,
I can only see data entered up to row 65536, the rest of the rows are empty. This of course ruins
my statistical processing (causes a #REF! message in every cell of the columns that I use in my statistical analysis), since it requires all the data to be present. I have seen that recent Calc versions can handle
up to 1million rows, but I cannot understand why my data vanishes when I reload my file. Any hints?

Store your File as native .ods and all will be fine.

( the .xls format stores only up to 65536 rows )

xlsx supports up to “Now with xlsx workbooks (and xlsm), the limits are 1,048,576 rows (220) and 16,384 columns (214) or the column XFD.” as told by http://www.excel-exercise.com/xls-xlsx-file/, so if one has to save as Excel, xlsx is way better option, but if not, then ods.

Thank you for the prompt response, I found about the .ods solution on my own after experimenting a little bit. It is a good thing that it also works with the .xlsx extension as you said, otherwise there would be serious
problem communication results to MS Office users. I tried that too and it works as you suggested.
Zillion thanks!

@enm1972: Mark the answer then, and if applicable upvote.