granulepos when concatenate two theora files

Hello! I need concatenate two files with theora video, without sound, to do
that i parse each file,
find packets inside, change flag 5 bit in packet header to 0 (continue flag)
and set for all packet
equal serial. This methon works fine, but have some problem with granulepos.
Help me please with
granule pos, i try many ways to set it right (set it like miliseconds for
example) but this not help.
Tell me please what is granulepos parameter and showind and how i need set
granulepos to do than time
 betwen each frame be 300 milisec. Thanks!

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