Overlapping Range Check

I think the easiest way to do this is to query the database and use a logical expression to check for 2 possible conditions:

1. An existing range starts before the start date you provided and ends after the start date you provided
2. An existing range starts before the end date you provided and ends after the end date you provided

If either of those conditions are true, then the ranges overlap. Here's a sample code to perform the query:

a!queryEntity(
  entity: cons!<YOUR ENTITY HERE>,
  query: a!query(
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: -1
    ),
    logicalExpression: a!queryLogicalExpression(
      operator: "OR",
      logicalExpressions: {
        a!queryLogicalExpression(
          operator: "AND",
          filters: {
            a!queryFilter(
              field: "startDate",
              operator: "<",
              value: ri!newStartDate
            ),
            a!queryFilter(
              field: "endDate",
              operator: ">",
              value: ri!newStartDate
            )
          }
        ),
        a!queryLogicalExpression(
          operator: "AND",
          filters: {
            a!queryFilter(
              field: "startDate",
              operator: "<",
              value: ri!newEndDate
            ),
            a!queryFilter(
              field: "endDate",
              operator: ">",
              value: ri!newEndDate
            )
          }
        )
      }
    )
  ),
  fetchTotalCount: false
)

I believe there is one additional scenario to consider, when an existing range starts after the start date and ends prior to the end date (provided dates surrounding completely).  All can be covered by reducing to one scenario:

1. An existing range starts <= to the end date you provided and ends >= the start date you provided

a!queryEntity(
  entity: cons!<YOUR ENTITY HERE>,
  query: a!query(
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: -1
    ),
    logicalExpression: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(
          field: "startDate",
          operator: "<=",
          value: ri!newEndDate
        ),
        a!queryFilter(
          field: "endDate",
          operator: ">=",
          value: ri!newStartDate
        )
      }
    )
  ),
  fetchTotalCount: false
)

Ah good call - you're right I missed one