thinkpic wrote:
I buy a stock for $200 on Sept 1. On Oct1 it is worth
$210. Obviously it has gone up by 5% over 30 days but
what would its current per annum percentage rise be.
The answere would be 5 divided by 30/365ths or aprox 60%.

That might not be the correct way to annualize stock
returns. Nonetheless, I can show you how to express
the formula that you have in mind.

This is even getting complicated for my math but I
have no idea how to express this problem as a formula
[....]
The only fields in the spreadsheet that I am hoping to
use a Column A = Purchase date; B = purchase price;
C = current price. (Hopefully the computer knows the
current date). Can this be done?

To answer your last question: yes, the TODAY() function
yields the current date.

However, I suggest that you put the date associated with
the "current" price into column D (e.g). Since you are
putting the "current" price into a cell -- not acquiring
it dynamically -- the computed growth rate will be
misleading if you use TODAY() and look at the spreadsheet
on another day.

Then the formula that you express above would be:

=(C1/B1 - 1)*(365/(D1-A1))

That yields 60.83% for your example, as you computed
manually.

That annualizes the amount of change. Some people might
argue that you should annualize the growth rate. The
formula for that is:

=(C1/B1)^(365/(D1-A1)) - 1

That yields 81.05% for your example.

Arguably, there are reasons why neither is the correct
answer.

1. Should you use the number trade days instead of the
number of calendar days?

2. Should you use the "square root of time" rule instead
of either a geometric or linear proportional change?

3. Is it misleading to annualize growth rates of smaller
time periods of a stochastic process?

I believe you will find champions of arguments on both
sides of each of those questions. I suggest that you
do a google search to decide which side you want to take.