Thats a estimate and isn't accurate it will go back further when you start up become it gains speed the longer its open.
How it is estimated ? Isn't it downloading a huge file of 20GB then it should be precise ?
Each time I re-open it it tells me that the database or something got corrupted.
No its not an estimate. Someone is confusing the workings of bitcoin core with those of a download. Its not a single 20 GB file either, but thats not important IMHO. Its the timestamp of the latest known block in comparisson to now. Each block has a (rougly estimate) timestamp when it was found. The time you see is how long ago in comparisson to your system clock this was.
Ill tell you what I allways say in these situations: use the torrent[1] or try bitcoin core 0.10.0rc3[2] as it supports a fast sync due to several improvements.
If you used the torrent connect[3] to a fast node for the rest. List of nodes:
last updated 2015.01.11
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IP - location - owner[1] - speed - info/stats page[2] - testnet
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84.200.34.113 - Freinsheim, DE, EU - Newar - 1000 mbit/s - yes - no
185.45.192.129 - Amsterdam, NL, EU - anon - 1000 mbit/s - /node.php[3] - yes
94.23.146.127 - NL, EU - anon - 200 mbit/s - /status - no
213.165.91.169 - Germany, EU - shorena - 100 mbit/s - yes - no
191.236.50.217 - Virgina, US - grue - 100 mbit/s - no - no
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!!The following nodes have been unreachable at the time of the last update and might[5] be down!!
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eldinhadzic.org[4] - LA, USA - eldinhadz - 1000 mbit/s - no - no
50.7.68.180 - New York, USA - Newar - 100 mbit/s - yes - no
5.9.24.81 - Germany, EU - zvs - unknown - no - no
178.79.173.71 - United Kingdom, EU - zvs - unknown - no - no
107.155.104.194 - Dalls, USA - zvs - unknown - no - no
106.185.32.195 - Japan, Asia - zvs - unknown - no - no
94.242.57.173 - Russia, Asia - zvs - unknown - no - no
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[1] refers to a bitcointalk.org username or anon if requested
[2] same IP, port 80 or path/port given
[3] work in progress
[4] IP is 173.236.158.177
[5] or reached max connections at the time of testing
[1]
https://bitcointalk.org/index.php?topic=145386.0 - there might be more recent bootstrap.dat sources
[2]
https://bitcoin.org/bin/0.10.0/test/[3] e.g. run c:\Program Files\Bitcoin\bitcoin-qt.exe -connect=1.2.3.4
