Would it be possible to leave at 1545 today?
or
Would it be possible to leave by 1545 today?
45 correctly. 45 is chiefly used in British English. 45 means you may leave even earlier.
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Both are correct if you write 15:45 / 15.45 correctly. 15.45 is chiefly used in British English. At 15.45 means you leave at that time, while by 15.45 means you may leave even earlier.
CB