Changing Member.premium_since back from Date to number fixes an error in the Discord electron client related to rendering premium status.

Client throws "Invalid time value", so I'm guessing it's something to do with premium_since not being the date format they want ( allegedly ISO8601, but works with a plain number, so wtf )

util/src/entities/Member.ts

@@ -86,7 +86,7 @@ export class Member extends BaseClassWithoutId {
 	joined_at: Date;
 
 	@Column({ nullable: true })
-	premium_since?: Date;
+	premium_since?: number;
 
 	@Column()
 	deaf: boolean;