If you used months as the base unit of a date scale X axis, Excel uses the
number of months since January 1900. If the X axis is a category type, Excel
uses 1 for the first category, 2 for the second, etc. If you want to use
dates, use real dates as the X values, and use a base unit of days, not
months (your display can show the months by using months for major and minor
units). This will give you as good accuracy as using an XY chart with dates
as the X value.

Your trendline formula does not display many digits. Use a scientific number
format with lots of digits, or as someone else has suggested, carry out the
calculations in the worksheet using LINEST.

- Jon
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Jon Peltier, Microsoft Excel MVP
Tutorials and Custom Solutions
Peltier Technical Services, Inc. - http://PeltierTech.com
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"tep" wrote in message
...
Using copy and paste I transferred the trendline equation (in this case a
quadratic) from a chart element to a worksheet cell and then subjected it
the
original data to validate it. The resulting values are magnitudes
different
to the orignal data - in the millions, not explained by regression
tolerance,
RHO of 0.998. The x axis values were months - what value does Excel assign
to
months in Charts because the 1900 system values (eg 39643 for 14 Jul 2008)
do
not appear to be ones used?