[R] about change columns for specific rows

[lily li]

Hi R users,

I have a data frame, and I generate a date column like this:
df$date = seq(as.Date('2012-01-01'), as.Date('2014-12-31'))

df
A  B  C
1  2   1
2  2   3
3  2   4

So the data frame has 4 columns now. But when I want to change the values
of column A for specific dates, such as 2012-01-01 to 2013-12-31, I use the
code below:
df[date >= '2012-01-01'&date <= '2013-12-31']$A =
df[date >= '2012-01-01'&date <= '2013-12-31']$A +2

But it does not work, the date I generate seems not effective. What is the
problem? Thanks for your help.

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