convert unix epoch time to human readable time

Question

First I have an Unix epoch time 1270787111.

Then based on this website: http://untangible.com/2009/01/covert-unix-epoch-dates-in-microsoft-excel-including-timezone-examples.html, I convert the number to be 40276.9758217592. Then I put the value into an Excel cell and convert it into "Date" format to be : 4/8/10 23:25 I do not know how this happens. How can the value 40926 be converted into 2010(year)04(month)08(day)? Is there any other programming way to convert an Unix epoch time into a human readable format time?

Answer 1

If you have the GNU coreutils date command, you can do this:

$ date -d @1270787111
Thu Apr  8 21:25:11 PDT 2010

If you'd rather do it in C, use localtime() or gmtime() to convert the time_t value 1270787111 to a struct tm, then use asctime() to convert that to a human-readable string. ctime(t) is equivalent to asctime(localtime(t)). This assumes that your C library represents time_t values as seconds since the Unix epoch (1970-01-01 00:00:00); POSIX guarantees this, but C does not.

strftime() takes a struct tm and converts it to a human-readable string, letting you control the format, rather than using the default "Thu Apr 8 21:25:11 2010\n" format imposed by ctime() and asctime() (yes, the default format includes a trailing newline).

Be sure to read the documentation for these functions. They require a lot of pointer manipulation.

Answer 2

My personal favorite is

perl -E 'say scalar gmtime 1270787111'

Answer 3

Excel starts dates from January 0, 1900 (yes, January 0). From this link.

The integer portion of the number, ddddd, represents the number of days since 1900-Jan-0

The date you end up with 40,276 days after 1/0/1900.

There are usually built-in functions in many programming languages (C/C++, Perl, PHP, etc.) for handling UNIX epoch timestamps. It really just depends on what language you're using as to what methods/functions you would call. Here's a link that shows some examples in C.