All of the above responses are correct. The easiest way I have seen to explain this difference is to use the following visual:
<------ Exposure Time (X) Marketing Time ----->
(X) = date of the valuation.
In other words, Exposure time is a look backwards as of the effective date of the valuation. The easiest way to determine it is to look at your comparable sales. Exposure Time estimates are a binding requirement in USPAP, and should best be expressed in the Reconciliation of Value.
Marketing Time is a forecast into the future as of the effective date of the appraisal, it's what might happen. The best way to make it is to examine the past and present, and go with your gut. Since there is no way to foretell the future, Marketing Time is not a binding requirement in USPAP, but rather a supplemental standard often asked for by clients.
In most cases for most properties, exposure time for most properties, marketing time and exposure time will generally be similar at any given time, except when the market experiences either a boom or a bust, sometimes attributable to a single event.
Whe it comes to explaining this to clients who usually don't really care anyway (they're just checking off a box on their review), the easiest way to handle this is to lump the two together in your report, like:
"This appraisal is based on an estimated Exposure Time/Marketing Time of 3-6 months", or words to that effect. I do this either in the Reconciliation or the Comments section, and anywhere else I express the value opinion in the report.
Of course, if you feel that there would be different estimates for the two, you should break them up and express them separately.
Hope this helps