I was attempting to create a flow that executed a GetItems action that would use the ODATA filter field to limit the rows to those where a date field (ProjectEnd) was less than or equal to the current date/time.
I accomplished this by creating a composed string variable that was the filter portion of what will be essentially a REST call to SharePoint. So if you're familiar with SharePoint REST call formats your goal will be to attain the same result and the query should work. In this case I needed the REST call to have $filter=ProjectEnd lt '2017-04-27T19:51:38.7610450Z'. That turned out to be tricky, but knowing that the browser would encode the single quotes anyway I just jumped ahead and did it in my variable. The resulting variable looks like this: ProjectEnd lt %272017-04-27T19:51:38.7610450Z%27
The result of this Compose can then be attached to the filter of the GetItems.
KEY: when you actually create this compose action item, put (double quotes) " around the value, they do not show once you've saved the flow but are critical to make it work.