I am now totally confused about what you require. What you tried is not what I suggested.
this is saying to me that if you have dal 1st Jan 2018 and al is 31st Jan 2018 you do not want any records returned where treatment started or ended between those dates - but you do want to see them if they started before 1st Jan and ended either before 1st Jan or after 31st Jan or they started (and ended_ after 31st Jan. Is this correct?
yes - use the form filter property, or rewrite the form recordsource each time.