Bitcoin Core Update from 0.13 to 0.15 and still verifying block, descending "new best chain" my blockchain was almost up to date and I wanted do update the Bitcoin core version. The database update was finished in roughly 30 minutes but now I'm stuck on verifying blocks.

Is this normal. How long can this take?

Now it is only repeating following message. But the date is descending?!? I saw other examples where it is ascending.

2017-10-20 15:06:14 Loaded best chain: hashBestChain=00000000000000000083cc7cd6e4643124170b7d69d95b38d17f65a63bb642f3 height=490620 date=2017-10-19 13:56:47 progress=0.998936
2017-10-20 15:06:14 init message: Verifiziere Blöcke...
2017-10-20 15:06:47 UpdateTip: new best=0000000000000000004615a7c3bf2258cb14bdf4b295d3d76e54c6f517779d3a height=490619 version=0x20000000 log2_work=87.310573 tx=263583692 date='2017-10-19 13:53:05' progress=0.998933 cache=1.3MiB(9552txo)
2017-10-20 15:07:26 UpdateTip: new best=00000000000000000041dbf43bd2725fc407bde4df841a9bfbbe3977d394bbb5 height=490618 version=0x20000000 log2_work=87.310534 tx=263581279 date='2017-10-19 13:50:08' progress=0.998930 cache=2.7MiB(19908txo)
2017-10-20 15:08:09 UpdateTip: new best=000000000000000000d16c73923d0ee22ee476f9bab5d4f06b907950b4e604ee height=490617 version=0x20000000 log2_work=87.310495 tx=263579133 date='2017-10-19 13:24:15' progress=0.998912 cache=3.9MiB(30408txo)
2017-10-20 15:08:50 UpdateTip: new best=000000000000000000cdf8fd2ab203fffa8efdce95b150a37171b5d71b479a38 height=490616 version=0x20000000 log2_work=87.310457 tx=263576112 date='2017-10-19 13:18:19' progress=0.998907 cache=5.3MiB(39669txo)
......
2017-10-20 19:29:31 UpdateTip: new best=000000000000000000c8365c1b69b14469257e64063f9fada6a37f5c9f21a9da height=488499 version=0x20000000 log2_work=87.229695 tx=259670084 date='2017-10-06 05:21:41' progress=0.985175 cache=334.7MiB(2186578txo)
2017-10-20 19:29:31 UpdateTip: new best=0000000000000000002c3caefefd142cca83f3edcae575ad663a1017656180df height=488498 version=0x20000000 log2_work=87.229656 tx=259669960 date='2017-10-06 05:21:39' progress=0.985175 cache=334.7MiB(2186636txo)
2017-10-20 19:29:41 UpdateTip: new best=000000000000000000b611b7693a1b26f763c25e8b0b1e66def99bdfa7910ff1 height=488497 version=0x20000000 log2_work=87.229618 tx=259667588 date='2017-10-06 05:08:27' progress=0.985166 cache=335.0MiB(2189168txo)
Bitcoin Core 0.13.0 did not have SegWit's activation parameters (they were added in 0.13.1). This means that after SegWit's activation, your full node was not aware of it, and thus did not ask for or verify block witnesses.

Bitcoin Core is a fully validating node, so after upgrading to 0.15 it noticed that blocks were present that were missing witnesses, and as result also didn't have those witnesses validated. As a result, it is rewinding blocks, undoing the effects of those post-SegWit-no-witness blocks back to SegWit's activation point. When that completes, it will start downloading the blocks again (with witness this time), and validating them.