Excel changes formatting of certain strings.

Hi dhAndrews and welcome to the forum,

If you start entering these numbers with an apostrophe (single quote) your data will be taken as text instead of converted to a date.

I occasionally work with CAS numbers as well, but I cannot recall having this same difficulty. I have participated in some discussions that suggest that different versions of Excel behave a little differently when it comes to automatically converting an entered string to date like this.

I wish there was a setting somewhere in Excel like "never auto convert values to dates". However, I am a little suprised that you indicate that Excel will eventually autoconvert these strings to dates in spite of the steps you have taken. I think it would be instructive if you could provide specific examples of when Excel will autoconvert these strings to dates. What you did to prevent the conversion, and what you were doing with the data when it autoconverted.

If interested, I am reminded of this discussion from earlier this year. We went round and round the problem until about post 26, where it was noticed that Excel 2016 appeared to be converting his text strings to dates during the processing (my copy of 2007 did not convert the strings to dates) http://www.excelforum.com/excel-gene...rediction.html . His solution was to not use "-" as his delimiter. In your case, CAS numbers have a long tradition of being displayed with "-" as the delimiter, so there is a stronger tradition here to use the "-". But it illustrates what I mean about how different versions of Excel can behave differently, and it illustrates what I mean by understanding exactly what you are doing when Excel auto-detects and converts to date.

I would note that you should be able to recover the text string after Excel converts to date using the TEXT() function, though it is really frustrating to us users of Excel to have to use such a "band-aid" when the real solution is for Excel to not autoconvert to date in the first place.

Anyway, all of that said, I would suggest that you provide some examples of exactly what you are trying to do when Excel autoconverts and let us see if we can help you devise a way for Excel to not mess up your CAS numbers.