This is not elegant, but it worked for me:
foos.SingleOrDefault(f => f.EntryDate.Year == bar.EntryDate.Year &&
f.EntryDate.Month == bar.EntryDate.Month &&
f.EntryDate.Day == bar.EntryDate.Day &&
f.EntryDate.Hour == bar.EntryDate.Hour &&
f.EntryDate.Minute == bar.EntryDate.Minute &&
f.EntryDate.Second == bar.EntryDate.Second);
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