Part 1. You don't want users to select end date before they select start date
Part 2. End date should always be greater than start date
1st is doable by using Dynamic Cell Access. What it will do is - it will check if your start date is blank then DCA remains unchecked and so does your End date line item (it will not have any dropdown). Below snips for you reference
As soon as you input your Start date, DCA becomes checked and so does your End date.
There is no way you can make Anaplan have end date greater than start date in input line item. If you have logic in place that should derive your End date then it can be coded accordingly.
As our other experts mentioned earlier, it is not possible to tackle your requirement with the help of Anaplan Time dimension. But you can still crack it by creating 'Custom Time'. Throwing my idea and check if it helps,
Step 1: Create Custom time list with 2 subsets (Always mark it for both).
Step 2: Use these subsets for your selection line items.
Step 3: Here we are trying to restrict the items in 'End date' thru Dependent Dropdown. Hence create a list called 'Dependent dropdown' and create an item in it.
Step 4: Convert the user selection to 'Date format' and apply the condition. Screen shots below for more clarification.