Validity period of e-visa
The e-visa is valid for 90 days upon approval, of which you can stay 30 days at most during your trip. The eVOA is valid for 30 days to enter Namibia and upon arrival you will receive a new period of 30 days of stay and with multiple entries. The e-visa is however a single entry visa, and the eVOA is a multiple entry visa.
Is your entry date earlier than the date specified on the questionnaire? Then you may not travel on the visa and must apply for a new visa Namibia before departure. Thus, entering later is not a problem, as long as you keep in mind that the maximum period of stay is then also shortened. Entering earlier than your specified date is not allowed.