How to calculate polynomial trendline x value when using a date?

Question

Level 1

4 points

How to calculate polynomial trendline x value when using a date?

So I have a chart that I have created in Numbers, with a trendline using a polynomial function, and I've got it to display the formula, but I cannot for the life of me, figure out what `x` value I'm supposed to use to get the resulting `y` value!

`x` is a date / time, so clearly it must be something related to the date value. But I've tried using ms since the unix epoch, secs since the unix epoch, etc. each to no avail. Some "other" spreadsheet tools make it clear how they convert dates into numbers when creating polynomial trendlines, Numbers however, does not.

Does anyone know what value I am supposed to use!?

For those that might not be able to read the graph easily, an `x` value of 01/01/2019 00:00:00 should give a `y` value of 1,000~.

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Posted on Apr 13, 2018 1:40 AM

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Answer 1

Siyfion Author

Level 1

4 points

Apr 13, 2018 4:17 AM in response to Yellowbox

2D Scatter and you're perfectly correct of course, if I convert the dates to a 0-based "days" then while it makes the graph slightly less easy to read (from an X-axis pov), it makes it MUCH easier to calculate. Thanks!

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Answer 2

Siyfion Author

Level 1

4 points

Apr 13, 2018 3:05 AM in response to Yellowbox

Hi Ian,

Thanks for your prompt reply, the data I'm using to generate the chart is really simple:

Then I'm using the Trendline - Polynomial option to create the line, with the "Show Equation" checkbox ticked.

What I really need to be able to do, is be able to get a Y value for a given date. The equation should give me this, but the problem is, X is a date, so I have no idea how to convert the date into a numeric that can be used in the equation! It's like I need to know how Numbers is converting it, to know how I need too...?

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Answer 3

Yellowbox

Level 7

22,045 points

Apr 13, 2018 4:16 AM in response to Siyfion

Hi Siyfion,

Numbers stores Date & Time values as serial numbers, but I don't know how to see what goes on "behind the curtain".

The only solution I can think of is to convert dates to number of days from the start. Using your data, I selected columns B and C and inserted a 2D Scatter