[FFmpeg-user] Why start_time is sometimes non zero?

[Andrzej Borucki]

I convert .mov file created by Canon to H.264, container mp4 - QuickTime(?)
ffprobe give me:
[format]
format_name=mov,mp4,m4a,3gp,3g2,mj2
format_long_name=QuickTime / MOV
start_time=0.045000
duration=7.808000
size=954931

video stream [streams.stream.0]
codec_name=h264
codec_time_base=1/180000
time_base=1/90000
start_pts=5940
start_time=0.066000
duration_ts=702702
duration=7.807800
bit_rate=810598
nb_frames=234

audio [streams.stream.1]
codec_name=aac
codec_time_base=1/48000
time_base=1/48000
start_pts=2160
start_time=0.045000
duration_ts=372736
duration=7.765333

After converting it to h264:
[format]
start_time=-0.033333
duration=7.842000
size=889221
bit_rate=907136

[streams.stream.0]
r_frame_rate=30000/1001
avg_frame_rate=30000/1001
time_base=1/30000
start_pts=0
start_time=0.000000
duration_ts=235235
duration=7.841167

[streams.stream.1]
time_base=1/48000
start_pts=-1600
start_time=-0.033333
duration_ts=374336
duration=7.798667

Why start_time is not zero? In second movie, audio begins before or after
start movie?
In first  - both video and audio start_time is non zero, what it means?