# How can I convert datetime vector to datenum (class-double)

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Farshid Daryabor on 28 Apr 2020
Answered: Peter Perkins on 30 Apr 2020
Please find attached file, time vector is original format in double I have, for some reason (preprocessing analysis) I have converted it to datetime format (Time) using below command. Again I want to convert Time to the original format (ie, class-double) according to the Pivot Year (1950-01-01). I really appreciate if someone can help me.
Time = datetime (time * 60 * 60 * 24, 'ConvertFrom', 'epochtime', 'Epoch', '1950-01-01');
Farshid Daryabor on 28 Apr 2020
Dear Ameer,
Thanks very good

Ameer Hamza on 28 Apr 2020
try this
pvtYear = datetime(1950, 01, 01);
time_ = datenum(Time-pvtYear); % time_ is same as time, minor differences probably due to floating point errors
Result:
>> all(ismembertol(time_,time))
ans =
logical
1
##### 2 CommentsShowHide 1 older comment
Ameer Hamza on 28 Apr 2020
Glad to be of help.

### More Answers (1)

Peter Perkins on 30 Apr 2020
You almost certainly DO NOT want to convert datetimes to datenums. In recent versions of MATLAB, there are not many good reasons to want to use datenum at all.
If you mean, "I need to convert back to elapsed time since 1950", that's maybe a different story, but those are not "datenums". Here's a simple way to do it:
dt = days(myDatetimes - datetime(1950,1,1))
However, if your values are counting days since 1950, you are shooting yourself in the foot -- you will never have an exact representation of time within day because 1/24, 1/1440. 1/86400, none of them are representable exactly in double precision.
Unless your timestamps are always at 00:00:00, 03:00:00, etc., you would be much better off using seconds.