如何在R中基于POSIXCt时间戳对data.frame进行子集

我想根据以下条件对以下data.frame进行子集

> head(df1)
        timeStamp g_id p_k_id weekday
1 2015-01-05 15:00:00     848            266     Mon
2 2015-01-05 15:01:00     848            266     Mon
3 2015-01-05 15:02:00     848            266     Mon
4 2015-01-05 15:02:00     848            266     Mon
5 2015-01-05 15:02:00     848            266     Mon
6 2015-01-05 15:03:00     848            266     Mon

> str(df1)
'data.frame':   33150 obs. of  4 variables:
 $ timeStamp     : POSIXct, format: "2015-01-05 15:00:00" "2015-01-05 15:00:00" "2015-01-05 15:00:00" ...
 $ g_id       : Factor w/ 2 levels "","848": 2 2 2 2 2 2 2 2 2 2 ...
 $ p_k_id     : Factor w/ 5 levels "","265","266",..: 3 3 3 3 3 3 3 3 3 3 ...
 $ weekday    : Factor w/ 4 levels "Fri","Mon","Sat",..: 2 2 2 2 2 2 2 2 2 2 ...

条件：

---

时间戳>=2015-01-05 15:01:00的行&可能类似于

df1[with（df1，timestamp>=“2015-01-05 15:01:00”&时间戳没错，谢谢@David Arenburg。
df2 <- df1[df1$timestamp >= 2015-01-05 15:01:00 & <= 2015-01-05 15:02:00]

Error: unexpected '<=' in...