Pythonic formatting and parsing of ISO 8601 ordinal dates.

iso8601_ordinaldate.py

# This snippet demonstrates Pythonic formatting and parsing of 
# ISO 8601 ordinal dates (4-digit year + "-" + ordinal day).
# Note: an ordinal day in this context is the nth day of 
# the year, with Jan 1 being ordinal day 1 and Dec 31 
# of non-leap year (a "common year") being day 365.

from datetime import datetime


d = datetime.strptime("2020-355", "%Y-%j")
print(format(d, "%Y-%j"))