as.POSIXct似乎正在产生NA，但我不确定为什么吗？
我尝试将format = "%Y/%m/%d %H:%M"更改为format = "%Y-%m-%d %H:%M"，但这没有用。我在这里想念什么？
df$`Local Time` <- as.POSIXct( df$`Local Time`,
format = "%Y/%m/%d %H:%M", tz="America/Vancouver")
数据
df <- structure(list(`UTC Time` = c("2019-06-01 10:03", "2019-06-01
10:07", "2019-06-01 11:41", "2019-06-01 13:13", "2019-06-01 13:53",
"2019-06-01 14:05"), `Local Time` = c("2019-06-01 3:03", "2019-06-01
3:07", "2019-06-01 4:41", "2019-06-01 6:13", "2019-06-01 6:53", "2019-
06-01 7:05"), Type = c("Passenger ship",
"Passenger ship", "Passenger ship", "Passenger ship", "Passenger
ship", "Passenger ship"), Length = c(294, 294, 294, 294, 294, 294)),
row.names = c(NA, -6L), class = c("tbl_df", "tbl", "data.frame"))