By default, each card is assigned Planned and Effective Start/End dates based on the card’s movement through the board and the Automatic Scheduling feature. The effective dates are meant to be automatically calculated by Elegantt but you can override their value at any time via the Advanced settings in the card edit popin.

Simply open the card edit popin and click on the arrow icon next to the Planned start date. You can then click on the Effective start/end dates to open the datepicker.