Tx_tag's time VS. USRP time

Hi everyone,
I wanna send data at desired time with USRP N210. The codes look like
these

int num=1;
pmt::pmt_t key = pmt::pmt_string_to_symbol(“sob”);
uint64_t offset = this->nitems_written(0);
pmt::pmt_t value = pmt::pmt_make_tuple(
pmt::pmt_from_uint64(5+3num),
pmt::pmt_from_double(0.0000000)
);
this->add_item_tag(0, offset, key, value);
num+=1;
offset = this->nitems_written(0)+n;
key = pmt::pmt_string_to_symbol(“tx_tag”);
value = pmt::pmt_make_tuple(
pmt::pmt_from_uint64(5+3
num),
pmt::pmt_from_double(0.0000000)
);
this->add_item_tag(0, offset, key, value);
num+=1;
offset = this->nitems_written(0);
key = pmt::pmt_string_to_symbol(“eob”);
value = pmt::pmt_make_tuple(
pmt::pmt_from_uint64(5+3*num),
pmt::pmt_from_double(0.0000000)
);
this->add_item_tag(0, offset, key, value);

It is obvious the code make the data sent at 8.000000s,11.000000s abd
14.000000s.On the receive side,I can see the data is sent every 3
seconds.This is correct.But I didn’t set USRP time to 0.000000 before
executing the code. This means the data were sent at (assuming it’s
300.100000s now) 303.100000s, 306.1000000s and 309.1000000. why this?
I’m using Ubuntu12.04 and gnuradio 3.6.5.1.
Any suggestion is appreciated.

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