To find the difference between dates, you want to use the {time: X} command which gives you the date in seconds.
Find the weeks from the selected date:
{formdate: YYYY-MM-DD; default=2020-09-30; name=date}
{ time: X} is the current date in seconds, { time: X; at={ =date}} is the selected date in seconds. We subtract one from the other to get the seconds between them.
We use "/ 60 / 60 / 24 / 7" to convert seconds into weeks.
Weeks to today from the selected date: {= ({time: X} - {time: X; at={=date}}) / 60 / 60 / 24 / 7; format=.2f}
"format=.2f" rounds to two decimal places. You could use "format=.0f" to round to a whole number.
Or if you wanted to hardcode the date rather than using a form selector:
{=({time: X} - {time: X; at=2020-09-30}) / 60 / 60 / 24 /7; format=.2f}