"since that" and "since then" both make sense and mean the same thing. "since then" seems preferable in careful English.
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CitiboxHello GPY, I am sorry but "since that" surely needs a qualifier? "Since that event"; "since he did so"; would do but just "since that" cannot be right.I think that "since that" is used by itself, isn't it? I think I use it sometimes. "that" is understood to mean "that time/event/incident/etc.".