We normally use "since" to mean "from a time in the past until a later past time, or until now".
Can we use "since" in future tenses, for example, "I will be free since 6pm today. Now is 3pm"?
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Yes we can use "since" with some future tenses, but "will" is not one of them. Future perfect, however, is:
I will have been free since 6 pm.
Though this isn't the most natural way to phrase the idea, the grammar is good. A more natural context would be one where the duration is being emphasized, not the starting point:
A: Let's go dancing after I finish work tomorrow.
B: No thanks. I'll have been on my feet since 9 am, so the last thing I'll want to do is dance.