You could either format the date string with Razor
@HttpUtility.UrlEncode(date)
with javascript
encodeURIComponent(date)
or pass the date as ticks (milliseconds since Epoch) instead of the human-readable format.
Edit:
After experimenting with this and a bit of research it seems the slash and %2f
encoding causes all kinds of problems. Stick to the millisecond representation for a date and not worry about passing the slash.