Hi @resteve,
Could you please explain a bit for where you woud like to put the filter, and what is this filter used for?
Besides, for the first day of this month, is it specially for "01/07/2017"?
If you would like to put the filter result within the items property of the BrowseGallery, then the suggested formula should be:
Filter('Testing list', Created>=Date(2017,7,1))
Things you need to know is currently < or > operator is not supported in delegation for SharePoint data source.
If they are used in the query formula, then you may notice the delegation notification, currenlty only "=" is supported delegation for SharePoint list.
For more details, please check:
Delegable data sources
Regards,
Michael