Time windows are indeed relative. The start point is always 0 minutes. Using ready time 180 and due time 360 is correct. Note that these need to be in an array “restrictions”, e.g.
{"name":"location1","lat":"51.33","lng":"4.18","restrictions":{"ready":180, "due":360}}
You may check the results using the website. The “Import” button can now be used to import the JSON you send to the API.
If you have doubts about a route, please share your input and the results here or send them via email info@routexl.com