Figure 1: A flow with an activity "Deadline Triggered" that will start when a deadline is trigger.
Figure 2: Edit the first activity "Do not complete to trigger deadline" to add a deadline shown in the screenshot. Set the "Duration unit" to "Date (dd/MM/yyyy)" and "Deadline limit" to a name of workflow variable. In this sample, the workflow variable will be "workflowVariable".
Figure 3: Edit the transition to set "Type" to "Exception" and "Expression" to the name of exception name set in the deadline.
Figure 4: Create a workflow variable.
Figure 5: Create a form with a date picker element to set a date value to workflow variable.
Figure 6: Edit the date picker's properties and set "Data format" to "dd/MM/yyyy"
Figure 7: Set the workflow variable