Three important dates to consider...
Michael, Dennis pretty much nailed this, but I think his language could have been a bit clearer. For that reason, I'll offer you something as explained by our Appraiser's Commission (as I remember it):
There are really three dates to consider in an appraisal. One of those is the date you see the property. It is usually not a USPAP requirement to report that date, but I find that it is often helpful to the reader in understanding things. Also, the new Fannie Mae 1004 form requires that the date of value (effective date) be the same day as the date the appraiser saw the property. Fannie wants to know what day you were there, but USPAP does not otherwise require you to report it unless it is necessary in order to not be misleading. For example, you might do your appraisal inspection on September 15th, but for a retrospective date of value on March 15th, and not finish the report until October 15th. In such a case, if you don't report the date you saw the property, the reader might think you saw it on the retrospective date, but such an appraisal should include an extraordinary assumption that the property was in the same condition on the date of value as on the date you saw it, unless it is otherwise described.
The second date to think about is the date of value, often called the effective date. This is important to report because the appraisal is valid as an opinion of value on a specific date. If you didn't report this date how would anyone know what your opinion of value was? The market is naturally a moving target and appraisals do not have a necessarily long shelf life.
The third date to think about is the date of the report. Most appraisers think of this as the date the report is signed. Steven Santora is correct in saying that you could sign the report early and then do the work later... but, is also correct is saying that it is not a very good work habit. Technically it may also be out of compliance because there is a reason the Appraisal Board wants to know the report date. They want to know what date you are considering the property from. (I know, you shouldn't end a sentence with a preposition.)
Anyway, the main reason the report date is important is because the USPAP and other rules that apply apply as of that date. If you are doing a retrospective value as of December 15, 2005 you would only consider data from the time period before that date. But, the USPAP that would apply to the report is the one in effect on the report date... today, in this example.
I usually think of the report date as being the day I came to my opinion of value. I may do additional clean-up work, and may actually physically affix the signature the next day, but I date it on the date I determined what my opinion was going to be. Knowing why the report date is important to the regulators makes this method seem most logical to me.
I believe your question, restated, was: Is it true that the date of value and the report date are usually two different dates?
I would say yes, that is usually the case. However, it is not mandatory.