Replicating DIFFPASTWEEK logic in BigQuery

My current view has a date column and a column with metric 1 (as shown in screenshot). In the Sisense widget front-end, I aggregate the dates as weeks. This sums up metric 1 for all the dates in a week.

I then use the DIFFPASTWEEK function to create a metric that shows the difference between each week. However, I do not understand the DIFFPASTWEEK function well enough and want to recreate this logic in BigQuery. Can anyone please advise on this? Thanks!