If possible use datetime instead of datenum. Let's look at two sample dates in different formats.
Dates = {'27-Jun-19', '13.04.2018'};
Try converting all the Dates using the first format.
DT = datetime(Dates, 'InputFormat', 'dd-MMM-yy')
The datetime function didn't know how to convert the second date using that format, so it returned NaT (Not-a-Time, the datetime equivalent of NaN or missing.) Let's identify the dates that could not be converted with the first format by looking for the NaT values.
otherFormat = isnat(DT);
Fill in the NaT values in DT by converting the corresponding elements in Dates using the second format.
DT(otherFormat) = datetime(Dates(otherFormat), 'InputFormat', 'dd.MM.yyyy')
This generalizes if you have a list of potential formats, just preallocate DT using the NaT function then convert the elements in Dates corresponding to NaT using each format in a for loop over the list of formats. Hopefully at each iteration of the loop the number of NaT values in your array decreases, so you'll have fewer and fewer dates to convert each iteration.
By the way, the reason you received that error in your original code is:
if G = 0
The = operator is for assignment, which is not legal as the condition of an if statement. The == operator tests two expressions for equality.
