If i use the example you attached (which use a constant as the reference line value);
-You can create a Calculated Field which returns the date you want to show on the reference line and on what condition.
-Next bring the Calculated Field on the Detail shelf. Then edit the reference line and select the calculated field instead of the Date measure.
When you say "I added reference lines to show separation at each month and year" , i don't know how you did that, because to have one line per month you would need to have the Date Dimension on the row shelf and set a line per Date cell.
Anyway i added some example which hopefully will give you ideas on what you can do with reference line.
Thanks, Michel! I was able to apply the calculated field for showing reference lines and this works great!