They can both carry the same meaning, but 'in' here to me suggests a much shorter time period within the overall time period. With 'during', it sounds as if the course is a full or half year and the workday is 8 hours (these are 'standard'). With 'in' it sounds as if the course is one quarter in length perhaps, and as if he works only 3 or 4 hours.
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You cannot use 'during' when you [can?] specify more particularly the time in the said time span.
'I was born during the summer of 1985' does not sound off. Why, do you suppose?