Time Since Chart Errors with Custom Date Field

Summary

Time Since Chart throws an internal server error when using a custom date field.

Steps to Reproduce

Steps to Reproduce:
1) Open a dashboard.
2) Add a "Time Since Chart" gadget to the dashboard.
3) Configure the dashboard to use a custom date field.
4) Click Save.

Expected Results

The gadget configuration should save and load correctly.

Actual Results

The gadget configuration does not save correctly and the user encounters an error message that says, "An internal server error occurred when requesting resource."