That's ok, IMHO. Here's how I see it: Imagine, you were to give a two-hour lecture at 17:25. Now it is 21:00.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
No one was to have given any additional information.
1. I was to start writing the letter yesterday at 8:00.
2. I was to have written the letter yesterday at 8:00.
The meaning of #1 is quite clear: the author planned to proceed to composing the letter at the indicated moment. As to #2, the speaker planned to finish the letter at 8:00 (or earlier, = "to have finished the letter by 8:00"). Am I right?
Here's