syncrepl giving error "CSN too old, ignoring " for detla replication

Hi,

I am using openldap 2.4.23-12.el6.i686 for provider and openldap version 2.2.28-4AV1.1 for slave. My configuration is Provider with proxy consumer on one machin and slave on another machine.

I am getting error “CSN too old, ignoring” in logs(provide-proxycosumer config)

slap_queue_csn: queing 0x8d7fe85e 20110901223155.770109Z#000000#000#000000

slap_graduate_commit_csn: removing 0x8d805020 20110901223155.770109Z#000000#000#000000

slap_queue_csn: queing 0x8b9fb300 20110901223155.770109Z#000000#000#000000

syncprov_sendresp: cookie=rid=128,csn=20110901223155.770109Z#000000#000#000000

do_syncrep2: rid=128 cookie=rid=128,csn=20110901223155.770109Z#000000#000#000000

do_syncrep2: rid=128 CSN too old, ignoring 20110901223155.770109Z#000000#000#000000

slap_graduate_commit_csn: removing 0x8d80b9a8 20110901223155.770109Z#000000#000#000000

Do my both provider and slave machines needs times synched using NTP or lag of few seconds is allowed (1-2 seconds)?

Thanks Rupesh