Converting unix time into date-time via excel

Question

Trying to convert 1504865618099.00 Unix time into a readable date time. I tried this:

=(UNIX + (\"1/1/1970\"-\"1/1/1900\"+1)*86400) / 86400

Answer 1

TLDR

=(A1/86400)+25569

...and the format of the cell should be date.

If it doesn't work for you

• If you get a number you forgot to format the output cell as a date.
• If you get ##### you probably don't have a real Unix time. Check your timestamps in https://www.epochconverter.com/. Try to divide your input by 10, 100, 1000 or 10000**
• You work with timestamps outside Excel's (very extended) limits.
• You didn't replace A1 with the cell containing the timestamp ;-p

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Explanation

Unix system represent a point in time as a number. Specifically the number of seconds* since a zero-time called the Unix epoch which is 1/1/1970 00:00 UTC/GMT. This number of seconds is called "Unix timestamp" or "Unix time" or "POSIX time" or just "timestamp" and sometimes (confusingly) "Unix epoch".

In the case of Excel they chose a different zero-time and step (because who wouldn't like variety in technical details?). So Excel counts days since 24 hours before 1/1/0000 UTC/GMT. So 25569 corresponds to 1/1/1970 00:00 UTC/GMT and 25570 to 2/1/1970 00:00.

Now please note that we have 86400 seconds per day (24 hours x60 minutes each x60 seconds) and you can understand what this formula does: A1/86400 converts seconds to days and +25569 adjusts for the offset between what is time-zero for Unix and what is time-zero for Excel.

By the way DATE(1970,1,1) will helpfully return 25569 for you in case you forget all this so a more "self-documenting" way to write our formula is:

=A1/(24*60*60) + DATE(1970,1,1)

P.S.: All these were already present in other answers and comments just not laid out as I like them and I don't feel it's OK to edit the hell out of another answer.

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*: that's almost correct because you should not count leap seconds

**: E.g. in the case of this question the number was number of milliseconds since the the Unix epoch.