diff --git a/calendar/generate_dates/app.py b/calendar/generate_dates/app.py
index b22893d..6e73221 100644
--- a/calendar/generate_dates/app.py
+++ b/calendar/generate_dates/app.py
@@ -4,56 +4,9 @@
 import sys
 
 from generate_dates import TeachingCalendar, config, templates
-from generate_dates.config import parse_command_line
-
-
-class PeriodError(KeyError):
-    pass
-
-class PaperError(KeyError):
-    pass
-
-class LectureStyleError(KeyError):
-    pass
-
-
-def parse_config(config, args):
-    """Validate and filter the top-level configuration.
-
-    Returns separate references to the period configuration and the
-    paper configuration, as applicable. (Yes, the period configuration
-    includes the paper configuration, but it may also include other
-    papers, so it's more convenient to return a separate reference
-    for the specified paper.)
-
-    The configuration file will normally contain specifications for
-    several teaching periods and papers. We don't need all of this:
-    usually we're only interested in one teaching period and one
-    paper (if relevant).
-    """
-    period_config = paper_config = None
-    # this is all irrelevant if we're going ISO style
-    if args.style != "iso":
-        if args.period not in config.keys():
-            raise PeriodError()
-
-        period_config = config[args.period]
-        # raises KeyError if source key is missing
-        period_config["name"] and period_config["weeks"]
-
-        # papers are irrelevant in "calendar" style
-        if args.style == "lecture":
-            if "papers" in period_config.keys():
-                if args.paper in period_config["papers"].keys():
-                    paper_config = period_config["papers"][args.paper]
-                    # we expect at least a "lectures" entry
-                    paper_config["lectures"]
-                else:
-                    raise PaperError()
-            else:
-                raise LectureStyleError()
-    
-    return period_config, paper_config
+from generate_dates.config import (
+    LectureStyleError, PaperError, PeriodError,
+    parse_command_line, parse_config)
 
 
 def run():