Leave existing data in a cell alone

I am trying to figure out a way to calculate data in a cell if the date is
current, or leave it alone (no operation) if it isn't. Basically I am using a
table to calculate test execution data. It would look something like the
following:

A B
1 96% 90% (data calculated on 3/1)

1 95% 89% (data Claculated on 3/2 in the same cells as above)

I then have another table that stores the data (manually right now) so that
I can graph it daily as follows:

A B C
5 3/1 96% 90%
6 3/2 95% 89%

What I was trying to do in B5 was =if(A5=TODAY(),A1,IF(A5<TODAY(),leave the
cell alone.

I can't seem to figure out a way to do this.

Help!!!

This will require circular reference, an advanced feature.
For ease of testing, this example uses 5 second intervals
instead of whole days.
Your data might look like this:
Meas
88
When Monitor
3/29/2006 16:44:00 89.00
3/29/2006 16:44:05 90.00
3/29/2006 16:44:10 91.00
3/29/2006 16:44:15 92.00
3/29/2006 16:44:20 92.00
3/29/2006 16:44:25 93.00
3/29/2006 16:44:30 94.00
3/29/2006 16:44:35 95.00
3/29/2006 16:44:40 96.00
3/29/2006 16:44:45 97.00
3/29/2006 16:44:50 98.00
3/29/2006 16:44:55

Name the cell <Meas> and the ranges <When>, <Monitor>.
Set Iterate to 1
Fill the <Monitor> column with this formula
=IF(AND(NOW()>=When,NOW()<When R[1]),Meas,Monitor)
It is written in R1C1 style.
For <When>, select a time ahead when you might start a test.
During the 5 second interval between 16:44:00 and 16:44:05
you can enter data into <Meas> and it will appear in <Monitor>.
At 16:44:05, this value will be locked into the first row of <Monitor>
and the next 5 second interval will be ready to sample <Meas>.
After everything works, revert to A1 and change the interval to one day.