Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
When the senate (in the spring of 704) at the suggestion of Pompeius
requested both him and Caesar to furnish each a legion
for the impending Parthian war(20) and when agreeably to this resolution
Pompeius demanded back from Caesar the legion lent to him
some years before, so as to send it to Syria, Caesar complied with
the double demand, because neither the opportuneness of this decree
of the senate nor the justice of the demand of Pompeius
could in themselves be disputed, and the keeping within the bounds
of the law and of formal loyalty was of more consequence to Caesar
than a few thousand soldiers.
The two legions came without delay
and placed themselves at the disposal of the government, but instead
of sending them to the Euphrates, the latter kept them at Capua
in readiness for Pompeius; and the public had once more the opportunity
of comparing the manifest endeavours of Caesar to avoid a rupture
with the perfidious preparation for war by his opponents.