Several functions in Google Scripts for Adwords supports the following date range strings:
TODAY, YESTERDAY, LAST_7_DAYS, THIS_WEEK_SUN_TODAY, LAST_WEEK, LAST_14_DAYS, LAST_30_DAYS, LAST_BUSINESS_WEEK, LAST_WEEK_SUN_SAT, THIS_MONTH, LAST_MONTH, ALL_TIME
I asked this same question on Google Groups (here is the answer.). To summarize, there is no public method for achieving this in Google Scripts, and a custom solution has to be deployed. Since I'm exporting the Adwords data to a personal database anyway I'll be solving this using PHP, so I won't be posting a solution here.