pySummerTime

# Find the dates of the start and end of Summertime in the UK, for any given year
# Find the minimum and maximum possible duration for Summertime this century
from datetime import *

def get_last_sunday(adate):
    d1 = datetime.strptime(adate, "%Y-%m-%d")       # make a datetime object
    dow = datetime.isoweekday(d1)                   # get day of week of last day in month
    dd = 31 - (dow % 7)                             # go back to the last Sunday in the month
    return adate[:8] + str(dd)                      # return full date of last Sunday

y = input("Enter a year from this century: ")

mindur, maxdur = 999, 0
for yy in range(2000, 2100):                        # loop for all years in this century
    ys = str(yy)
    summerstart = get_last_sunday(ys + "-03-31")    # gat date of last Sunday in March
    summerend = get_last_sunday(ys + "-10-31")      # get date of last Sunday in October
    dstart = datetime.strptime(summerstart, "%Y-%m-%d")     # construct datetime objects
    dend = datetime.strptime(summerend, "%Y-%m-%d")
    duration = abs((dend - dstart).days)            # compute difference in dates
    mindur = min(mindur, duration)                  # get min duration so far
    maxdur = max(maxdur, duration)                  # get max duration so far
    if ys == y:                                     # print results for a specific year
        print(f"Summer time in {ys} is from {summerstart} to {summerend} and lasts for {duration} days")

print(f"In years from 2000 to 2099, Summertime can last between {mindur} and {maxdur} days")