The difference between my answer dgallup's comes down to the design of a title block and how many date entries are available and how they are labeled.
The title blocks I use (which were designed by me) contain the following:
"Drawn By" box in which the designer puts their employee number (some people us initials. I don't like that. employee numbers are unique, initials are not) which is input automatically via PDM.
Below that box is a box labeled 'Date' into which the designer or engineer puts the date the drawing is released at rev A (or rev 0 or whatever system you use). Once the drawing is released, this date does NOT change, ever.
Revisions beyond A are captured in a revision block which contains a single line per revision with revision code, employee number, short description, and date. Our PDM system automatically generates everything except the short description. The date is not editable. The system captures the date (and time, though this extra information is stored in PDM under the rev level only and does not appear on the drawing)
The way our system works (and the way it was constructed before we implemented PDM and all of this was done manually) is the same as described by dgallup- the system automatically generates a date code for each rev, but only when the rev command is run. The drawing can be opened as many times as necessary ad nauseam and until the rev command is run, no date codes are altered.
Looking at the 'date modified' value in the file system tells me nothing. If someone moves the file to a new folder, that date changes. If someone renames the file or whatever, it changes. That value is useless.
You have to look at things from the customer point of view- as a customer, I simply do not care when a drawing was 'started'. With regard to what is captured on a drawing, I do not care about how long it took the designer to develop the design. All I care about is that when I order a part, I'm ordering the correct revision and that a newer one is not available. So all I care about is the date of initial release (meaning the date the part became saleable to me) and the dates of any subsequent revisions.
Setting up the system so that the 'drawn on' date, or any other date, is altered every time the drawing is opened is a bad idea. If I opened a drawing from a customer and the 'drawn on' date is later than any of the revisions, once again I am sending that drawing back for clarification.
