Python Max Function in Field Calculator for Date values

Question

I have 4 fields with Date Time values, and created a new field (Date type as well) which takes the most recent date from these 4 fields. I have tried using the Max python in the field calculator to populate this new field, but it does not use the most recent date from these 4 fields. Does this function not work with Datetime values, or is there another function for this that I am not able to find on these forums.

max([!field1!, !field2!, !field3!, !field4!])

I am on 10.5. When I have two fields that have a date within the same month, it will not read the rest of the date correctly. (See picture attached)

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When I run the following updated code it fails and says it is a syntax error.

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With the current code, it reads the first number in the day to select the most recent date. (As shown below). It does not use the correct chronological format, even though the strptime format worked previously. enter image description here

• Alex Tereshenkov: It works for me ( max( [!Date1!, !Date2!] ), but max( !Date1!, !Date2! ) also works. I am on ArcGIS 10.5. Which version do you use? Also, what kind of values do you have in your fields, please post. Also, make sure all of them are of Date type.
• Purplepeopleeater: I am also on 10.5, but when I have two fields that have a date within the same month, it will not read the rest of the date correctly. (See picture attached)
• Alex Tereshenkov: Which field is the result of calculation? Which field do you expect to be used for max?
• Midavalo: Can you please show the code you have used (as text) and also the column names for your screenshot that you added today? The code in my answer doesn't look at days or months etc., but at a datetime object, so it should just be returning the max value. If it's not then there may be an issue with the data or you've missed the field in the code? It's hard to tell without a bit more.

Answer 1

You need to convert your date into datetime object for max() to work correctly. It appears field calculator doesn't see the value as a datetime object. You may need to adjust the date format to suit your region date settings. See Strptime Behavior in the Python docs.

Using the Python Parser, select Show Codeblock. In the Pre-logic script code:

from datetime import datetime
def dtmax(*args):
 f = '%m/%d/%Y %I:%M:%S %p'
 dlist= list()
 for d in args:
 if d:
 dlist.append(datetime.strptime(d, f))
 return max(dlist)

Expression:

dtmax(!DateField1!, !DateField2!, !DateField3!, !DateField4!)

• Paul: Would be nice if this was generalized as dtmax(*args) instead of requiring 4 fields exactly.
• Midavalo: @Paul Good idea, updated.
• Purplepeopleeater: With args I do not need to list the fields?
• Midavalo: @Paul That's correct, and how I had first written it while testing, however when learning I prefer things written out as (in my opinion) it can make things a bit clearer as to what is happening in the code. Once Python is more understood by the user, then we can look at other ways to write it.
• Midavalo: @Purplepeopleeater Please give more info. Edit your question and add a description and some detail about what is happening now. Don't remove anything from your existing question, just add to the bottom of it.