In Excel, if your timestamp column is at A, then prepare a new column for this formula: =(A1/86400)+DATE(1970,1,1)+TIME(8,0,0)

The +TIME(8,0,0) is for my particular GMT+8 timezone in asia, adjust it to your own if you like, or omit the whole thing for UTC.

Then format the new column as a Date. I chose Custom with this format string: yyyy-mm-dd hh:mm:ss