[R] Aggregating an its series

[Vittorio]

I'm using a very long irregular time-series of air temperature and 
relative humidity of this kind (this is an extract only)

its.format("%
Y%d%m %X)
> base
                                 T  H
20020601 
12.00.00 27.1 47
20020601 15.00.00 29.1 39
20020601 18.00.00 27.4 39
20020601 21.00.00 24.0 40
20020602 0.00.00  22.0 73
20020602 3.00.00  
19.2 49
20020602 6.00.00  19.5 74
20020602 9.00.00  24.8 45
20020602 
12.00.00 27.7 NA
20020602 15.00.00 29.2 39
20020602 18.00.00 27.2 44
20020602 21.00.00 23.9 50
20020603 0.00.00  21.0 75
20020603 3.00.00  
19.6 65
20020603 6.00.00  19.8 71
20020603 9.00.00  23.2 67
20020603 
12.00.00 24.9 65
20020603 15.00.00 21.7 74
20020603 18.00.00 22.8 63
20020603 21.00.00 21.2 75
20020604 0.00.00  18.0 91

I would like to 
aggregate T and H by day and produce another its of this kind

> aggr
               T   H
20020601 NA NA
20020602 NA NA
20020603 NA NA
20020604 NA NA

where the daily average of T and H (not counting the NA 
values) is is put in aggr.

What is the quickest way to get this 
result?

Thanks

Vittorio