The Bereznikers

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"