What do the dates on a recently issued Thai visa mean?

My partner recently applied for a Thai tourist visa. She expects to arrive on date A, and to leave 5 weeks later on date B - we correctly filled in the application to reflect this (we have photocopies!).

Her passport was returned with two dates on the visa: date C and date D. Date C appears to be the day the embassy received the application (and it is already in the past), and date D is someway between her expected stay dates.

In pictorial form:

                            A                 B
Dates applied for:          [-----------------]
Dates on visa:        [------------]
                      C            D

So, what do the dates on the visa represent?

• Do they represent the expected stay applied for, and are therefore in error?
• Do they represent a range of dates within which she can enter Thailand, with her exit data being assigned when she does so?