Expression for the "Interval" input of a schedule step cannot be evaluated or its syntax needs to be clarified

**Describe the bug**
For a "Schedule" step, the expression for "Interval" created through Json/Yaml cannot be evaluated.

**To Reproduce**
An exception is thrown when we use "datetime.timedelta(seconds=300)" as the "Interval" input.

**Additional context**
The scenario is using the REST API for creating a workflow definition by passing a JSON representation; afterwards, the "Workflow" API is used to start/create a workflow. The exception is thrown when the input expression for a schedule step is evaluated.

If I am wrong on defining the "Interval" expression, please correct me. Thanks for your great work.