Call admissions to be sure, but I recall transcripts only being considered official if they are sealed individually by school. If you put them all into one envelope and seal it yourself, they have no way of confirming you didn't tamper with the documents first or that they were even generated by your school. Some schools are now eligible to send transcripts to UT electronically and have them considered official, but I think that might only be for other college transcripts. Your school should know whether or not they offer this option.
There isn't usually a date discrepancy issue, but I suppose it depends on how your school seals it's transcripts. All of mine had a stamp with a signature across the flap on the back of the envelope, but no date to compare it to when the transcript was prepared. I doubt it would be a concern, but you could always ask admissions to be sure if you look at the transcript at the seal across the back does contain a date.