Note that the SUM function automatically ignores text.
Regarding dates, it depends on how they are entered into Excel.
Valid date entries in Excel REALLY are just numbers with special date formats applied (dates are stored in Excel as the number of days since 1/0/1900).
So any valid dates would be included in the SUM, but if they are entered as Text, they will not be.
In your example, since April has the word "From" in it, it is a text entry, so would not be picked up by the SUM function.
So the SUM function would return exactly 18 on your sample data.