OpenOffice Calc only opens, but doesn't save xlsx.
LibreOffice Calc also saves as xlsx, but the result will probably look weird with Excel.
If you want to get the best possible compatibility, I recommend using PlanMaker from SoftMaker. It has the best import and export filters for Microsoft Office formats and allows seamless interoperability.
PlanMaker is a part of SoftMaker Office which is commercial, but available for a fair price; if you don't want to spend any money, you can try out SoftMaker's Lite-Version FreeOffice which costs nothing, plus, btw., is much faster and smaller than OpenOffice and LibreOffice:
freeoffice.com