@GAlbano76  As @jesusbc mentioned, a timestamp is simply the number of seconds (milliseconds) since the unix epoch > January 01, 1970 UTC.
Conversions to humanreadable dates can be done as follows
(examples in Python, R and MS Excel):
Python:
import pandas as pd
pd.to_datetime(1501621140000, unit=‘ms’, utc=True)
R: (processes timestamps in seconds, divide ms by 1000)
as.POSIXct(1501621140000 / 1000, origin=“19700101”, tz=“UTC”)
MS Excel:

Enter your timestamp (1501621140000) in let’s say, cell A1

Remember it’s seconds from 01011970, so you need to:
 Divide by 1000 to convert to seconds,
 Then divide by 86,400 (the number of seconds in 1 calendar day)
 Then add the date (January 01, 1970) to the result
… to get the date corresponding to the timestamp.
You can do this easily by pasting the following formula in a cell (in this case C1, with the timestamp in A1):
=(A1/1000)/86400+DATE(1970,1,1)
This will first give you a numeric value in your cell. You’ll need to convert the cell format to Date in Cell > Format Cells, to get this: