I'm writing a report on runway quality and I've stumbled onto a weird qualifier in the data. I understand that runway 17/35 indicates which direction the runway is facing based on magnetic heading.

RWY_ID 17/35 17/35 26-Aug 14/32 30-Dec 28-Oct 21-Mar 13/31

What does 21-March mean for example?

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    $\begingroup$ Where did you come across this data? Context might help. Please Edit your question. $\endgroup$ – a CVn Oct 5 '18 at 12:22

Very likely, whatever data source you're using has been mangled by Excel (or some other spreadsheet or data processing system that tries to convert everything that might be a date into a date format).

26-Aug is actually runway 26/08, 30-Dec is actually runway 30/12, etc. (See "How are runways numbered?")

Some computer system saw those numbers, assumed that they were dates, and presented them that way. The other runways aren't valid as dates, so it left them alone. It wouldn't be the first time that people used Excel without fully understanding its default date-parsing settings and thereby causing problems for people trying to use their data afterward.

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    $\begingroup$ Brilliant deduction! $\endgroup$ – Michael Hall Oct 5 '18 at 19:27
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    $\begingroup$ 100% Excel. No other software is stupid enough to automatically convert cells differently even though they are in the same column. $\endgroup$ – Eric Duminil Oct 6 '18 at 13:19
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    $\begingroup$ To emphasize the seriousness of the problem in @Peter Cooper's answer, the original data is not just formatted differently, it is gone! A cell that contained 10/28 and another cell that contained 10-28, will now be equal, with both now containing 43401 (at least now in October, 2018) $\endgroup$ – DJohnM Oct 7 '18 at 7:02
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    $\begingroup$ Must have been an American Excel as the runways have the smaller number first, so 08/26, 12/30, 10/28 and 03/21 $\endgroup$ – KayEss Oct 8 '18 at 11:16
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    $\begingroup$ Good that he was no about to paint those numbers on runways :) $\endgroup$ – PeterM Oct 8 '18 at 16:01

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