Previous versions of HEC-RAS followed HEC 6, removing dredged mass instantaneously, in the time step closest to the Dredge Event Start Date/Time. Users could specify staged dredging with multiple dredging events, but this was tedious and still discontinuous. Instantaneous dredging is still the default method. All of the dredge methods only require one date. However, HEC-RAS 5.0 includes the option to distribute dredging over a time window, but pairing a Dredge Start Date with a Dredge End Date. This distributes the prescribed dredging over a time window.
The Dredge End Date was added to use with the four Mass dredge methods. Dividing a specified dredged mass over several time steps, removing proportional mass from each one is straight forward. Dredging end dates are a little more complicated in association with the Elevation methods, because it has to update the scour rate to account for cross section aggredation or degredation over the dredge window, to arrive at the specified elevation at the end of the dredge window.