Start with the teaser's description. Many of them carry a release line written as Year.Month.Day, and some add a time of day while others do not. Copy that line down exactly as written, because it is the claim we are going to check. Think of it as a promise about the day only. If there is no time attached, the teaser has not told you the hour, and it is better to leave that part blank than to guess one.
On the date itself, look for a new upload with the same song title and an [MV] prefix in the title. That prefix marks the full video, while a [Teaser] prefix marks the short preview. Open the video page and read the upload time listed there. That time is stamped by the platform, so it is the one to trust. The date line in the teaser only gave you the day, and the platform stamp is what lets you say when the video actually went up.
The teaser and the MV do not always sit on the same channel. One may be on the act's own channel and the other on a distributor channel, so check both before you decide anything is missing. A distributor channel may also carry its own notice saying it is an official channel for the MV and that music shows count the views there. That is worth reading, because it tells you the upload is an official one. Once the MV is up, its description may add credits or an album name that the teaser lacked, but it also may not, so do not expect extra detail.
If the MV does not appear on the day, do not assume a delay or a cancellation. The platform may hold the video for a premiere, or the post may be on another channel you have not looked at yet. Check the act's own announcements before drawing any conclusion. Keep the honest limits in mind too. Date lines come in different formats, the date may be given in a different time zone from yours, so the day can shift for you, and a teaser alone is never a guarantee of the exact hour.