I have a Pandas dataframe that looks like:

```
   YYYYMMDD  HH
0  19900101   1
1  19900101   2
2  19900101   3
3  19900101   4
4  19900101   5

```

With 
YYYYMMDD: year-month-day values as integers. 
HH: hourly values as integers. 

I want to get a column 'DateTime' which is a datetime column as yyyy-mm-dd hh:mm:ss. (so, mm:ss=00:00).

```
df['DateTime'] = df['YYYYMMDD'].apply(lambda x: pd.to_datetime(str(x), format='%Y%m%d'))

```

Gives me yyyy-mm-dd as datetime, is there a fancy way to add 'HH' values to the datetime?