The effective start date of the contract.
When this is using a fixed date model, it will be the same for each asset.
When using a sliding date model, this will be the minimum start date across all assets associated with the contract.
If the contract has a Fixed date model, this value is editable and required.
If the contract has a Sliding date model, this value is non-editable. It will instead
be a retrieval-only property.