How to get the day difference between date-column and maximum date of same column or different column in Python?
Although I was busy with this question for 2 days, now I realized that I had a big mistake. Sorry to everyone. 
The reason that can not take the maximum value as date comes from as below.
Existing one: t=data_signups[["date_joined"]].max()
Must-be-One:  t=data_signups["date_joined"].max()
So it works with as below.
data_signups['joined_to_today'] = (data_signups['date_joined'].max() - data_signups['date_joined']).dt.days
data_signups.head(3)
There will be no two brackets. So stupid mistake. Thank you.
I am setting up a new column as the day difference in Python (on Jupyter notebook). 
I carried out the day difference between the column date and current day. Also, I carried out that the day difference between the date column and newly created day via current day (Current day -/+ input days with timedelta function). 
However, whenever I use max() of the same column and different column, the day difference column has NaN values. It does not make sense for me, maybe I am missing the date type. When I checked the types all of them seems datetime64 (already converted to datetime64 by me). 
I thought that the reason was having not big enough date. However, it happens with any specific date like max(datecolumn)+timedelta(days=i). 


t=data_signups[["date_joined"]].max()


date_joined   2019-07-18 07:47:24.963450
dtype: datetime64[ns]


t = t + timedelta(30)


date_joined   2019-08-17 07:47:24.963450
dtype: datetime64[ns]


data_signups['joined_to_today'] = (t - data_signups['date_joined']).dt.days
data_signups.head(2)


shortened...

date_joined_______________//  joined_to_today________
2019-05-31 10:52:06.327341  //  nan
2019-04-02 09:20:26.520272  //  nan

However it worked on Current day task like below.


Currentdate = datetime.datetime.now()
    print(Currentdate)
    2019-09-01 17:05:48.934362
before_days=int(input("Enter the number of days before today for analysis ")) 
30


Done


last_day_for_analysis = Currentdate - timedelta(days=before_days)
print(last_day_for_analysis)


2019-08-02 17:05:48.934362


data_signups['joined_to_today'] = (last_day_for_analysis - data_signups['date_joined']).dt.days
data_signups.head(2)


shortened...

date_joined_______________//  joined_to_today________
2019-05-31 10:52:06.327341  //  63
2019-04-02 09:20:26.520272  //  122

I expect that there is datetype problem. However, I could not figure out since all of them are datetime64. There are no NaN values in the columns.
Thank you for your help. I am newbie and I try to learn everyday continuously.