How can I sum values over a range of dates in Numbers?
It works by dereferencing the value of a cell directly to the condition

Example:

SUMIFS(Daily Log::Income,Daily Log::A,"&gt;=2014-07-01",Daily Log::A,"&lt;2014-08-01")


Referencing the cell could also be made possible with the same construct; by adding ampersand followed by the cell name outside the double quotes.

Example:

SUMIFS(Daily Log::Income,Daily Log::A,"&gt;="&amp;A3,Daily Log::A,"&lt;"&amp;A4)