The Unix time stamp tracks time as a running count of seconds. The count begins at the "Unix Epoch" on January 1st, 1970, so a Unix time stamp is simply the total seconds between any given date and the Unix Epoch. Since a day contains 86400 seconds (24 hours x 60 minutes x 60 seconds), conversion to Excel time can be done by dividing days by 86400 and adding the date value for January 1st, 1970.
In the example shown, the formula first divides the time stamp value in B5 by 86400, then adds the date value for the Unix Epoch, January 1, 1970. The formula evaluates like this:
=(B5/86400)+DATE(1970,1,1) =(1538352000/86400)+25569 =43374
When C5 is formatted with the Excel date "d-mmm-yyyy", the date is displayed as 1-Oct-2018.
How Excel tracks dates time
The Excel date system starts on January 1, 1900 and counts forward. The table below shows the numeric values associated with a few random dates:
|1-Oct-2018 12:00 PM||43374.5|
Notice the last date includes a time as well. Since one day equals 1, and one day equals 24 hours, time in Excel can represented as fractional values of 1, as shown in the table below. In order to see the value displayed as a time, a time format needs to be applied.