need to develop CSPRNG. can i use ANSI X9.31 generator need to create secure PRNG. can i use ANSI X9.31 generator and modify its entropy rather using DT use something else >?
Yes, you can use any generator from NIST SP 800-90A rev.1 (PDF) as SEJPM indicated in the initial comment above.



Adding entropy is called seeding a random number generator. Seeding a PRNG with just date/time is sinful at best. Using a nano-second precision clock is only somewhat better; generally anybody can guess the value of the time so date / time itself is not random enough for secure random number generators.

Yes, you can seed using /dev/urandom which itself is a PRNG, seeded by the operating system. You could also use /dev/random (without the u) to retrieve a seed more directly from the entropy pool of the operating system. However, using /dev/random may cause performance issues and /dev/random may block as well. In general using /dev/urandom should be preferred.

If you use at least 128 bits of randomness from the /dev/urandom then your qualified PRNG should be at least as secure as using /dev/urandom directly. The security of /dev/urandom of course relies on the implementation, configuration and indeed the quality of the entropy it relies on. You may also seed the date / time in addition to the 128 bits of entropy, but date / time brings so little benefit that you might as well skip that part.