Cuts current trace to given start time. For more info see trim().
Example
>>> tr = Trace(data=np.arange(0, 10))
>>> tr.stats.delta = 1.0
>>> tr._ltrim(tr.stats.starttime + 8)
<...Trace object at 0x...>
>>> tr.data
array([8, 9])
>>> tr.stats.starttime
UTCDateTime(1970, 1, 1, 0, 0, 8)