Q:

How to convert excel date to timestamp

I got array of dates from excel file and they look like strange fractional numbers: 42425.4166550926, 42419.3326041667, 42411.5419097222, etc. How to convert them to unix timestamp or javascript date?

A:

To convert date to timestamp, a formula can work it out.
Select a blank cell and type this formula =(A1-DATE(1970,1,1))*86400 into it and press Enter key, if you need, you can apply a range with this formula by dragging the autofill handle. Now a range of date cells have been converted to Unix timestamps.

Source
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