Create Missing Timecards Between Two Dates

There are elegant solutions that you could try for this - but it comes down to the goal with uncovering what is missing and will this be a one time thing? From reading your note - it sounds like going forward you plan to use a pipeline to create the missing ones so maybe this is only a question of getting the gaps. 

In regards to backfilling, you could do a couple things: 

1. Just do it in Excel and then build an import to load historicals. You can just index your users with each week of the year and see if they have a timecard for that week and then use that to make an upload. This assumes that the backfill is only a one time thing. 
2. Make a summary report of your users and group by week to count the number of timecards. If the user has one it will show (1), blanks/null values means its missing. There is no direct action but its a simple way to see what's missing and where and then import the blanks or send it out to users to fill in whats missing. 
3. This is the most complex but solves the short and long term: Create a table for 'Employee Weeks' - the table would be a join between a user and each week in the user. You would make the 'key' field a string concatenation of email + week (so like jdoe@email.com-1/1/23). You can then make a relationship to your timecards where you join the user and week as a formula field so that they link up automatically. Using your basic summary fields you can check if the employee has as time card or not and report and send out emails accordingly to those missing. You would pre-load this table with an import of your employees and whatever time period you want to check, and since you can chart out each week until the end of time you can import several years worth of these records all at once to check historically or into the future.