Are both choices possible? Do they mean different things? Also, is it okay to omit “by” and “at”?
I’ll have finished this work by/at this time tomorrow.
1. I’ll have finished this work by this time tomorrow. 2.
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1. I’ll have finished this work by this time tomorrow.
2. I’ll have finished this work at this time tomorrow.
3. I’ll have finished this work this time tomorrow.
(2) and (3) mean the same, but (2) is not tremendously natural. Instead of (2) we normally say (3).
(1) means that "finished this work" will happen sometime between now and this