calculate number of weeks between today and a start date

How about this?

=($A$5-C11)/7

An alternative means is with Power Query which seems a bit more convoluted but is here only as a learning option.

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Task ", Int64.Type}, {"Column1", type any}, {"start date ", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Today", each DateTime.Date(DateTime.LocalNow())),
    #"Added Custom1" = Table.AddColumn(#"Added Custom", "Custom", each [Today]-[#"start date "]),
    #"Added Custom2" = Table.AddColumn(#"Added Custom1", "Custom.1", each [Custom]/7),
    #"Changed Type1" = Table.TransformColumnTypes(#"Added Custom2",{{"Custom.1", Int64.Type}}),
    #"Removed Columns" = Table.RemoveColumns(#"Changed Type1",{"Today", "Custom"})
in
    #"Removed Columns"

Power Query is a free AddIn for Excel 2010 and 2013, and is built-in functionality from Excel 2016 onwards (where it is referred to as "Get & Transform Data").

It is a powerful yet simple way of getting, changing and using data from a broad variety of sources, creating steps which may be easily repeated and refreshed. I strongly recommend learning how to use Power Query - it's among the most powerful functionalities of Excel.

- Follow this link to learn how to install Power Query in Excel 2010 / 2013.

- Follow this link for an introduction to Power Query functionality.

- Follow this link for a video which demonstrates how to use Power Query code provided.