You have multiple options as this is what I call a "selective sum" problem (You want a sum or other aggregate value, but only for some of the available records).
A calculation field using ExecuteSQL could show this total via a Join and also a Where clause.
A summary field defined in your timesheet field can be placed inside a one row portal on your job layout with a portal filter that limits the records summarized to those of the current month.
You an set up a relationship to a new table occurrence of your Timesheet layout and use an additional match field that matches the current month to a month field set up in your timesheet table.
Here's a sample calculation that could be used for both match fields:
DateFIeld - Day ( DateFIeld ) + 1
This computes a date for the first day of the month for all dates in the same month. In the Jobs table, make this an unstored calculation and substitute Get ( CurrentDate ) for "dateField". In your timesheet table, this would be a stored calculation that references the date field that records the date of the timesheet record's hours.