If you ever find yourself needing to convert from UNIX time in Excel, here's the solution.

Unix time is the number of seconds since January 1, 1970.

Excel doesn't contain built-in functions for working with Unix dates so they must be derived.
Excel allows you to add a number of days to a date by using the "+" operator. Let's make use of that.
First convert the number of seconds to number of days (by dividing by 60*60*24) and then add the result to the date "1/1/1970".
The formula will look like

=CELL/(60*60*24)+"1/1/1970"

The quotes around the date are required. If they are not present, Excel will treat 1/1/1970 as an expression.

On Linux (or any other OS that uses coreutils), you can type

date +%s

to see the current unix time.

=CELL/(60*60*24)+"1/1/1970"