[R] Adding 1 month to a dataframe column

[ANGELO.LINARDI at bancaditalia.it]

Dear R experts,
I have a problem in modifying one column of a dataframe with a datatime
format using a datetime operator.
Here is my dataframe A:

     DATACONT PROVINCIA VALORE
1  2007-12-31        MI      1
2  2007-12-31        PV      2
3  2007-12-31        NA      3
4  2007-12-31        MI      4
5  2007-12-31        RM      5
6  2007-12-31        RM      6
7  2007-12-31        MI      7
8  2008-12-31        MI     11
9  2008-12-31        PV     12
10 2008-12-31        NA     13
11 2008-12-31        MI     14
12 2008-12-31        RM     15
13 2008-12-31        RM     16
14 2008-12-31        MI     17
15 2006-12-31        MI     -9
16 2006-12-31        PV     -8
17 2006-12-31        NA     -7
18 2006-12-31        MI     -6
19 2006-12-31        RM     -5
20 2006-12-31        RM     -4
21 2006-12-31        MI     -3

Suppose I want to shift all dates by 1 year, obtaining the following
dataframe B:

DATACONT PROVINCIA VALORE
1  2008-12-31        MI      1
2  2008-12-31        PV      2
3  2008-12-31        NA      3
4  2008-12-31        MI      4
5  2008-12-31        RM      5
6  2008-12-31        RM      6
7  2008-12-31        MI      7
8  2009-12-31        MI     11
9  2009-12-31        PV     12
10 2009-12-31        NA     13
11 2009-12-31        MI     14
12 2009-12-31        RM     15
13 2009-12-31        RM     16
14 2009-12-31        MI     17
15 2007-12-31        MI     -9
16 2007-12-31        PV     -8
17 2007-12-31        NA     -7
18 2007-12-31        MI     -6
19 2007-12-31        RM     -5
20 2007-12-31        RM     -4
21 2007-12-31        MI     -3

I tried with seq.Date:
seq.Date(a$"DATA",by="1 month",len=2)[2]

but it works only for single values (the "from" must have length=1).
Any hints ?
Thank you in advance

Angelo Linardi

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