The first definition below produces the warning in the title when compiled with f# 3.0 and the warning level set to 5. The second definition compiles cleanly. I wondered if someone could please explain just what the compiler worries I might accidentally mutate, or how would splitting the expression with a let clause help avoid that. Many thanks. let ticks_with_warning () : int64 = System.DateTime.Now.Ticks let ticks_clean () : int64 = let t = System.DateTime.Now t.Ticks