I have a data file of temperature reading with date stamps, and I need to plot various graphs where the indexes from 1h00 to 5h00 are removed. To this point, everything works fine, using a formula to identify the undesirable date stamps and replace by a void.

F2=(Date stamp)
=IF(AND(MOD(F2;1)<0,25;MOD(F2;1)<>0);"";F2)
The resulting column is exactly what I need, and plotting a long range of it, with the associated temperature reading should result in a few curves separated by 4-hours gaps...

The thing is that I could not find a way to properly set a void. Using "" in the formula is treated as a zero, and thus it reads "jan 1, 1900". Only manually deleting the "content" of the voided cells allows me to get the right graph. Considering +50k lines of data, I would like to avoid doing that!

I included a sample file with 3 graphs; the first one uses only data with dates, so the plot is just fine. The second graphs shows a plot where the void cells have been manually "cleaned" with delete, and still shows an acceptable result (but can't do with the hole file!)
When unchanged, using void cells as obtained with the formula in column C, the graph is all wrong, using the 0 date index (jan 1 1900).

TEST.xls

We tried both in Excel 2003 and 2010 with the same results.

Ideas anyone?

Thanks,

Annie