Gazette announcements and their date of being effective

Announcements in the Royal Gazette have at least two dates - the date when it was published in the Gazette (วันที่ประกาศ), and the date it was approved and signed and thus deemed ready for publication. Sometimes the publication follows few days after the approval, sometimes a whole year expires between the two dates. Some announcements carry a third date, the one when the announcement gets effective, usually with the last sentence of the announcement being something like ทั้งนี้ ตั้งแต่วันที่ ... เป็นต้นไป.

But what about those announcements which lack this date - are they effective with the date of approval, or the date of publication? Or can they be effective at a date not mentioned in the announcement at all? For the recent municipality creations the file I just got contains a column named วันที่มีผลใช้บังคับ (date effective), which is apparently identical with the approval date in the announcement.

The upgrades of minor districts to full districts never include the effective date. See for example the upgrade in 1997 - the announcement was published on September 26, it was signed September 15, but amphoe.com for several of the districts claims these were updated on October 11: ต่อมาได้รับการยกฐานขึ้นเป็นอำเภอหนองปรือ ตั้งแต่วันที่ 11 ตุลาคม 2540 เป็นต้นมา (from here). I simply wonder how comes this date is not mentioned in the announcement, and if it is real how to find this date for all those announcements where it wasn't stated.