The Date of Separation is the day the spouses stopped living together with at least one spouse intending the separation be permanent. Generally, the date of separation is more important in marking the day the marriage ended than the actual divorce decree.
Impact
Property Division (Equitable Distribution): When dividing marital property and debt 50/50, in most cases the Court is looking at the value of the property or debt as of the date of separation. This is effectively when the joint venture called marriage ended. Financial decisions and spending after that point are the responsibility of each individual spouse.
Alimony/Child Support: The date of separation is the farthest back the court can go for retroactive child support and alimony.
Divorce: The date of separation is the day the timer starts for the one year of separation that is required before you qualify to file for divorce.
Criteria
The date of separation is the date[1]:
One spouse moves out of the marital home
At least one of the spouses intended for the separation to be permanent
The parties do not reconcile or continue the marital relationship in such a way that would make it appear to others that they are still together[2]
Living in separate rooms does not start the date of separation. The spouses must live in different homes all-together. However, separate apartments in the same building is enough to trigger the date of separation, unless the parties are still behaving like a married couple. Behaving like a married couple can include things like sharing accounts, cooking together, and sharing household responsibilities, even if there is no sex or intimacy between the parties.[3]

