user7: Is it possible to calculate a timestamp from a slot no? Since one slot is one second it should be pretty easy. But...
The current slot now (Tue 18 May 2021 05:46:04 PM CEST) is 29786450.
In mainnet-shelley-genesis.json i can see systemStart  "2017-09-23T21:44:51Z"
Which is acually 1506203091 as unix timestamp.
If i calculate 1506203091 + 29786450 i get 1535989541 which is Sep 3, 2018, 5:45:41 PM something in 2018.
Is there a simple formular? It seems that the actual start date must be something like June 7, 2020 but i cannot find anything abount this date.

user1147: Calculating a timestamp from a slot number is risky!
Theoretically, the slot duration could be changed by a hard fork for example because of performance reasons (as has already happened). So it's probably best to use an oracle for now.
But because time is used so frequently, it would probably be best if Cardano would support a more reliable native solution that avoids oracle fees.

user4360: Note this answer could change if a hard fork changes parameters.
The Shelly unix time is the start time of the Shelly era and the Shelly slot is the corresponding slot. We use these values to ignore the Bryon era (with odd slot value).
Calculate a slot: (returns the Cardano slot)
UNIX_TARGET_TIME - 1596491091+ 4924800

Check a slot: (returns unix timestamp)
1596491091 + (SLOT - 4924800)

Python example:
from datetime import datetime

SHELLY_UNIX = 1596491091
SHELLY_SLOT = 4924800

def getSlotFromUnixTime(ut):
    return ut - SHELLY_UNIX + SHELLY_SLOT

def getUTCEstimateFromSlot(slot):
    ts = SHELLY_UNIX + (slot - SHELLY_SLOT)
    print(datetime.utcfromtimestamp(ts).strftime('%Y-%m-%d %H:%M:%S'))

source code: https://github.com/pixel-pool/PythonCardanoSlotsScript

user28: You did not account for Byron, where one slot was 20 seconds.
To make it easier, take the start time of epoch 209 which is the Shelley hardfork and started at 1596491091 unix time on slot 4924800.
Therefore to calculate the time of the current slot, you might try something like: 1596491091 + (current_slot - 4924800).

7: <p>Is it possible to calculate a timestamp from a slot no? Since one slot is one second it should be pretty easy. But...</p>
<p>The current slot now (<code>Tue 18 May 2021 05:46:04 PM CEST</code>) is <code>29786450</code>.</p>
<p>In mainnet-shelley-genesis.json i can see <code>systemStart  &quot;2017-09-23T21:44:51Z&quot;</code></p>
<p>Which is acually 1506203091 as unix timestamp.</p>
<p>If i calculate <code>1506203091 + 29786450</code> i get <code>1535989541</code> which is <code>Sep 3, 2018, 5:45:41 PM</code> something in 2018.</p>
<p>Is there a simple formular? It seems that the actual start date must be something like <code>June 7, 2020</code> but i cannot find anything abount this date.</p>
 1147: <p><strong>Calculating a timestamp from a slot number is risky!</strong><br>
Theoretically, the slot duration could be changed by a hard fork for example because of performance reasons (as has already happened). So it's probably best to use an oracle for now.</p>
<p>But because time is used so frequently, it would probably be best if Cardano would support a more reliable native solution that avoids oracle fees.</p>
 4360: <p>Note this answer could change if a hard fork changes parameters.</p>
<p>The Shelly unix time is the start time of the Shelly era and the Shelly slot is the corresponding slot. We use these values to ignore the Bryon era (with odd slot value).</p>
<p><strong>Calculate a slot:</strong> (returns the Cardano slot)</p>
<pre><code>UNIX_TARGET_TIME - 1596491091+ 4924800
</code></pre>
<p><strong>Check a slot:</strong> (returns unix timestamp)</p>
<pre><code>1596491091 + (SLOT - 4924800)
</code></pre>
<p><strong>Python example:</strong></p>
<pre><code>from datetime import datetime

SHELLY_UNIX = 1596491091
SHELLY_SLOT = 4924800

def getSlotFromUnixTime(ut):
    return ut - SHELLY_UNIX + SHELLY_SLOT

def getUTCEstimateFromSlot(slot):
    ts = SHELLY_UNIX + (slot - SHELLY_SLOT)
    print(datetime.utcfromtimestamp(ts).strftime('%Y-%m-%d %H:%M:%S'))
</code></pre>
<p>source code: <a href="https://github.com/pixel-pool/PythonCardanoSlotsScript" rel="nofollow noreferrer">https://github.com/pixel-pool/PythonCardanoSlotsScript</a></p>
 28: <p>You did not account for Byron, where one slot was 20 seconds.</p>
<p>To make it easier, take the start time of epoch 209 which is the Shelley hardfork and started at <code>1596491091</code> unix time on slot <code>4924800</code>.</p>
<p>Therefore to calculate the time of the current slot, you might try something like: <code>1596491091 + (current_slot - 4924800)</code>.</p>
