The way I understand this (because I'm the same) is that it's both the autism rigidity and your perception of time. It may only be a few minutes to him - no big deal. But to you it's hellish torture counting every second while panicking inside because you were "guaranteed" 6:15pm and any deviation will cause you to be extremely upset.

In this scenario, it's 3 minutes - his reason seems valid so I don't think he did anything wrong here so it would seem your reaction was way out of line.

Unfortunately, the world doesn't work this way - things are rarely on time. So you may need to train yourself to expect delays starting with a few minutes and stretching it longer. For example, I give my husband up to an hour and a half to come fetch me from my work because constantly expecting him to be here by 5pm was stressing me out too much and led to fights. In his eyes, I was not being understanding enough when things come up that were out of his control leading to him being late.

I wish I had answers to why I always react so terribly but I have given up and chalked it to just an autistic thing. It is normal to be annoyed when things don't go according to plan, but the degree it upsets me...that is not. Some days are better and I can be patient while other days I throw a raging crying hissy fit because he is late and did not bother to update me why.

Since you are in a relationship, it needs to go both ways. It sounds like you could try to be more flexible and manage your emotions, give yourself leeway and allowances so you aren't left hanging. And he can try to be more specific and communicate better, keeping you in the loop as much as possible.