问当我运行这段代码时，图表中没有发生任何事情。

EN

// This source code is subject to the terms of the Mozilla Public License 2.0 at https://mozilla.org/MPL/2.0/
// © jithuldevaraj01

//@version=5

strategy ("VRD's MA Crossover Strategy v1", overlay = true)
start = timestamp (2007, 1,1,0,0)
end = timestamp (2021,6,1,0,0)
ema1 = ta.ema(close, 20)
ema2 = ta.ema(close, 50)
plot (ema1, title = "EMA20", color = color.blue,)
plot (ema2, title = "EMA50", color = color.green)

//strategy
LongEntry = ta.crossover (ema1, ema2)
LongExit = ta.crossover (ema2, ema1)
ShortEntry = ta.crossover (ema2, ema1)
ShortExit = ta.crossover (ema1, ema2)

if time >= start and time < end
    strategy.entry ("Long", strategy.long, 1, when = LongEntry)
    strategy.close ("Long", when = LongExit)
    strategy.entry ("Short", strategy.short, 1, when = ShortEntry)
    strategy.close ("Short", when = ShortExit)