Datetime values are much better than datenum values. Nevertheless, here's how to convert your datestrings to both.
% Date strings of format "yyyymmddHHMMSS.SS"
dates={'20020804122604.66';'20020804122610.66'}
% Convert to datetime
dtm = datetime(dates,'InputFormat','yyyyMMddHHmmss.SS');
% Convert to datenum (if you must; not recommended)
dnm = datenum(dtm)
With the datetime values, you can also specify the output format,
dtm = datetime(dates,'InputFormat','yyyyMMddHHmmss.SS','Format','yyyyMMddHHmmss.SS');
Datetime arrays are much more useful than datenum values and are the primary way to store and represent date and time data in Matlab. There are many core functions that support datetime arrays that become difficult to perform with datenum values. Arithmetic and datetime comparisons are much easier with datetime values. You can specify time zones and specify the human-readable format of datetime values (including POSIX time). Datetime values are especially useful when dates and times are plotted since they appear as date/times in the axis ticks.
Nearly none of the capabilities mentioned above support datenum values.
