The "Automatically update" check box in the Insert | Date and Time dialog
applies only to a date that you insert from that dialog when that box is
checked or unchecked. It has no bearing on previously inserted dates. If you
use the Alt+Shift+D shortcut or the Insert Date button on the Header and
Footer toolbar, either will insert a DATE field. If you don't want the field
to update, insert a CREATEDATE field instead (or edit the DATE field to
change it to CREATEDATE). A CREATEDATE field is what you should have in your
template as well. If you insert a date field in error, you can unlink it
using Ctrl+Shift+F9, but this will not restore the original date if the
field has already updated; only changing it to CREATEDATE will do that.
For more on date fields, see
http://word.mvps.org/FAQs/TblsFldsFms/DateFields.htm