Pre-filled DateTime field incorrect in graph when coming from Dashboard

A negative from param results in an incorrect DateTime being prefilled in the "From" field.

eg Url to graph from Dashboard:  http://domain:port/graphs/type=device_bits/period=86400/device=1/to=now/from=-86400/draw_all=yes/legend=no/class=image-refresh/

How to Reproduce:

• Go to device url: http://domain:port/device/device=1/
• At top of page, click "Traffic" graph
• On traffic graph page (device_bits), click "Add to Dashboard"
• Go to Dashboard
• Click on traffic graph that was just added to dashboard
• From DateTime is pre-populated incorrectly

If you remove the negative from parameter from the url, then it functions as per normal due to the presence of the period and to param.

Note: This issue is not isolated to the device_bits graph