How do I add leads and lags for my TWFE regression with month-year data?

Question

I have data which looks like this:

N = 10

ISIC <- c(111, 112, 113, 114, 115, 111, 112, 113, 114, 115)
n_workers <- rnorm(N)
year_month <- c(201801, 201801, 201802, 201801, 201802, 201901, 201902, 201901, 201902, 201903)
year <- c(2018, 2018, 2018, 2018, 2018, 2019, 2019, 2019, 2019, 2019)
month <- c(01, 01, 02, 01, 02, 01, 02, 01, 02, 03)

df <- data.frame(ISIC, n_workers, year_month, year, month)

How do I add leads and lags for the variable n_workers based on both ISIC, month and year in this situation? So, the 1 year lead ISIC == 111 for 201801 should be ISIC == 111 in 201901.

Thank you.