Hi globustrek,
This problem is caused by the inserted date being generated by an unlocked DATE field. If you select the errant date and press Shift-F9, it will probably look something like:
{DATE} or {DATE \@ "dd/MM/yyyy"}
To set the date back to the date the document was created, change the 'DATE' string to 'CREATEDATE', so that the field looks something like:
{CREATEDATE} or {CREATEDATE \@ "dd/MM/yyyy"}
Then select the field again and press F9 to update it.
Other semi-permanent DATE field options you might want to explore include changing the 'DATE' string in the field to:
SAVEDATE and PRINTDATE
which, as their names suggest, display the date on which the document was last saved or printed, respectively. Neither of these fields returns a valid date if the document hasn't been saved or printed, as the case may be.
Cheers