Hi Evan thanks for posting in the Lucid Community! With Timelines like your Gantt chart you are limited to full units of your chosen time delineation - e.g. full days full weeks or full months. For this reason it's not possible to set a block to span 1.5 weeks if your timeline is subdivided by weeks. In this case I would recommend adjusting your timeline's division to days so that you can more precisely indicate the length of block. In this example below I've set a task to span a week and half by choosing the precise days: 

I hope this helps! However I can understand the need to keep the divisions in weeks while indicating more precise start and end dates and I've logged this idea for review by our development team which it will utilize when prioritizing improvements to Lucidspark. If you'd like to share additional detail about your use case and why this would be beneficial to you we'd love to hear more either in this thread or via this feature request form. Thanks for your feedback and apologies for any inconvenience! Please let me know if you have additional questions.