Python - Filter a list of string dates which are in different formats

For a list of datetime objects, use List Comprehensions directly to filter based on the condition.
Below example filters the dates after 2020 and then converts to a string format.

from datetime import datetime

initialDates = [datetime(2020, 10, 15, 0, 0), datetime(2021, 12, 22, 0, 0), datetime(2022, 1, 18, 0, 0), datetime(2021, 10, 15, 8, 40, 50), datetime(2018, 8, 14, 22, 35)]

filtereddates = [dt for dt in initialDates if dt.year > 2020]
filtdates = [datetime.strftime(dt, '%Y/%m/%d %H:%M:%S') for dt in filtereddates]
print(filtdates)

Output: ['2021/12/22 00:00:00', '2022/01/18 00:00:00', '2021/10/15 08:40:50']

String dates list in same format

Convert the list to datetime objects list using strptime() and then apply filter condition.

from datetime import datetime

#Date in string format
datelist = ['2020/10/15', '2019/10/15', '2021/10/15', '2022/02/05']
#Convert to datetime objects list
dtlist = [datetime.strptime(dt, '%Y/%m/%d') for dt in datelist]

#Filter conditions
filtereddates = [dt for dt in dtlist if dt.year > 2020]
#Convert output to string format, if needed
filtdates = [datetime.strftime(dt, '%Y/%m/%d %H:%M:%S') for dt in filtereddates]
print(filtdates)

Output: ['2021/10/15 00:00:00', '2022/02/05 00:00:00']

String dates list in different format

For a list of string dates in multiple formats, loop through each item and use a try...except block to find the correct format.
Then apply the required filter.

from datetime import datetime

def formatDates(datestosort):
    formatlist = ['%Y/%m/%d', '%m-%d-%y', '%y.%m.%d', '%m-%d-%Y %H:%M:%S', '%Y.%m.%d %H:%M']

    newdtlist = []
    for dt in datestosort:
        for fl in formatlist:
            try:
                newdtlist.append(datetime.strptime(dt, fl))
                break
            except:
                pass
        
    return newdtlist

#Input
datelist = ['2020/10/15', '12-22-21', '22.01.18', '10-15-2021 08:40:50', '2018.08.14 22:35']
#Convert string dates to datetime
dtlist = formatDates(datelist)

#Filter conditions
filtereddates = [dt for dt in dtlist if dt.year > 2020]
#Convert output to string format if needed
filtdates = [datetime.strftime(dt, '%Y/%m/%d %H:%M:%S') for dt in filtereddates]
print(filtdates)

Output: ['2021/12/22 00:00:00', '2022/01/18 00:00:00', '2021/10/15 08:40:50']