# Add up millisecond steps to a certain datetime

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Karl Drenker on 15 Dec 2021
Edited: Siddharth Bhutiya on 30 Dec 2021
I try to add milliseconds to a certain datetime. In dtStart.Var1 the times are in milliseconds (25,50,75, ...). I would like to add this to my current time_of_day. With my code, however, in the end I lose the milliseconds when converting back to datetime.
DateVector = datevec(time_of_day);
A = (dtStart.Var1/1000)+DateVector(:,6);
C = cat(2,DateVector,A);
B = C(:,[1,2,3,4,5,7]);
time = datetime(B,'InputFormat','yyyy-MM-dd HH:mm:ss.SSS');

Chunru on 15 Dec 2021
Use "Format" for output (not "InputFormat" which is for input).
DateVector = [2021 12 15 01 30 05]
DateVector = 1×6
2021 12 15 1 30 5
A = DateVector;
A(6) = (345/1000)+A(:,6) % ms: 345
A = 1×6
1.0e+03 * 2.0210 0.0120 0.0150 0.0010 0.0300 0.0053
time = datetime(A, 'Format','yyyy-MM-dd HH:mm:ss.SSS')
time = datetime
2021-12-15 01:30:05.345
Karl Drenker on 15 Dec 2021

Siddharth Bhutiya on 30 Dec 2021
Edited: Siddharth Bhutiya on 30 Dec 2021
You don't need to convert the datetime into a datevec. You can convert your millisecond values into a duration using the milliseconds function and directly add it to the datetime.
>> dt = datetime(2021,12,(1:5)',"Format","dd-MMM-uuuu HH:mm:ss.SSS")
dt =
5×1 datetime array
01-Dec-2021 00:00:00.000
02-Dec-2021 00:00:00.000
03-Dec-2021 00:00:00.000
04-Dec-2021 00:00:00.000
05-Dec-2021 00:00:00.000
>> millisVals = [100;200;300;123;456];
>> dt = dt + milliseconds(millisVals)
dt =
5×1 datetime array
01-Dec-2021 00:00:00.100
02-Dec-2021 00:00:00.200
03-Dec-2021 00:00:00.300
04-Dec-2021 00:00:00.123
05-Dec-2021 00:00:00.456