I understand you want to use from config..you cannot put string.format but you can put what is there inside it in config
And {0},{1} etc can be used to replace calues you need
If you want to control today yesterday ..10 days before you can do that also by adding alogiv like adddays and pass days from config
Odata filter by default is just a filter but not something which would calculate expressions..what you see on sharepoint list also id you are selecting today but if in url it would canculate date and add it..that is how it would work
Cheers