Having issue with consolidating date and time vectors in large array to datetime in one vector

Date is typically (in Excel) a whole number where 1 represents 1/1/1900, and time is the floating point fraction of a day

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What you are telling me is that to concatenate the two columns requires (1) that the date reference needs to be consistent to the same date between logger file (consistent with Microsoft)and the MatLab software definition of date relative to a starting date (different than Microsoft date). Then I must convert the time into a real fraction and add this to the integer date, as one final output vector.
That sounds about right, as I did run into some odd looking date times after trying straightforward addition without conversion to Matlab date.

That's pretty much it. Matlab does have functions for manipulating dates and times:
However, if it were me; I'd upload into Excel and do the concatenation there.

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I was using Excel previously when the files only had about 35,000 rows. Since we will be running for about 1 week to 2 weeks at a time, the LOGGERXX.csv might have as many as 1,209,600 rows as some of the data is updated once per second, some has a variable update time about 5 seconds to 60 seconds, depending on a dial setting, and some updates once per minute. Plotting part of all these separate blocks of data into one consolidated graphic might get interesting. We have not had the ability to get everything running on one Arduino Mega. Trying to work more than about 35,000 rows in Excel gets really tedious, and I think it cannot handle past 1,000,000 rows anyway.