there is a report to see which employee didn't fill "Weekly Timesheet" in X period of time?

Use an Employee saved search, use a formula field to get the maximum date of time records. then use that same filter to set as Criteria for employees whom don't have a value in the period of time you are setting.

I couldn't find in here formula for max, also in NetSuite docs it not mentioned, do you know which one I should choose?

Many thanks!