df[3:13] <- sapply(df[3:13], as.numeric)
df$YearMonth <- strftime(df$date, "%Y %m")
  #Get last available date in a month
LastDate <- function(x){
  y <- filter(x, !is.na(x))
  last(y)
}

LastDate(df[1:20,1])

df %>%
  select(China:US, YearMonth) %>%
  group_by(YearMonth) %>%
  summarise_all("LastDate")

x <- 1:10
LastDate(x)
x[10] = NA