# How to do basic time arithmetic when NaNs are involved

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Blue on 24 May 2020
Commented: Blue on 24 May 2020
Hi,
I have the following table and I simply want to calculate the elapsed time between start and end points. My main problem is that I dont know how to handle the NaNs. The desired output is elapsed_time (in hours).
start_date = {NaN, '2019-05-21 00:00:00', '2019-05-22 00:00:00', '2019-05-23 00:00:00'}'
start_time = {'13:00', NaN, '01:00', '04:00'}'
end_date = {'2019-05-20 00:00:00', '2019-05-21 00:00:00', NaN, '2019-05-23 00:00:00'}'
end_time = {'14:00', '03:00', NaN, '05:00'}'
T = table(start_date, start_time, end_date, end_time)
% Desired output
T.elapsed_time = {NaT, NaT, NaT, '01:00'}'
Thank you,

Steven Lord on 24 May 2020
Replace the NaN numeric values in your date cell arrays with 'NaT' and the NaN numeric values in your time cell arrays with 'NaN'. When I did this in release R2020a I was able to convert the dates to datetime arrays and the times to duration arrays by calling datetime and duration (with a value for the InputFormat option) respectively. Then just add and subtract them using + and -.
Blue on 24 May 2020
Hi Steven,
Im not too sure how you are doing that replacement bit. Something along the lines of this ?
idx = cellfun(@isnan, T.start_date, 'UniformOutput', false)
T.start_date(idx) = 'Nat'
idx = cellfun(@isnan, T.end_date, 'UniformOutput', false)
T.end_date(idx) = 'Nat'
idx = cellfun(@isnan, T.start_time, 'UniformOutput', false)
T.start_time(idx) = 'NaN'
idx = cellfun(@isnan, T.end_time, 'UniformOutput', false)
T.end_time(idx) = 'NaN'

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