>>> dfDLA.reset_index()['ETD'] - dfDLA.reset_index()['STD']
0   0 days 00:30:00
1   0 days 00:50:00
2   0 days 00:20:00
3   0 days 00:30:00
4   0 days 02:00:00
5   0 days 01:30:00
dtype: timedelta64[ns]


>>> dfDLA['DLA'] = dfDLA.reset_index()['ETD'] - dfDLA.reset_index()['STD']


>>> dfDLA['DLA']
STD                  FLT
2021-05-13 07:00:00  418   NaT
2021-05-13 07:05:00  302   NaT
                     401   NaT
2021-05-13 07:15:00  404   NaT
2021-05-14 06:50:00  407   NaT
2021-05-14 07:00:00  418   NaT
Name: DLA, dtype: timedelta64[ns]

왜 NaT로 나오는 걸까요? 인터넷 계속 찾아보다 현기증 나서 물어봅니다