This might help you:
Specifically this section:
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Relative to some other anchor date
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two weeks before Sep 28 2015
%ICUDateTimeFor%TIME(2015, 9, 28-14)%EEE, MMM d, yyyy%
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Week 20 of 2016
%ICUDateTimeFor%TIME(2016, 1, 1+19*7)%EEE, MMM d, yyyy%
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Six weeks after Apr 20 2016
%ICUDateTimeFor%TIME(2016, 4, 20+6*7)%EEE, MMM d, yyyy%
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I can’t be more specific as I’m a long way from my desk! Anyway - hope it helps.