There is no difference in meaning, and 'it' remains existential. 'When' is much more usual.
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AnonymousIstheirthere any difference in the meaning of the two sentences ?
AnonymousNo.
Istheirthere any difference in the function of "it" in the two sentences ?
Debpriya DeBut I was under the impression that the "it" in the first sentence is a "dummy it" whereas the "it" in the second sentence is an anticipatory it referring to the noun clause "that I left the office".