I am not sure if this works, but I am asking anyways 
I want my ESP8266 to attempt to connect to a WiFi network for 30 Seconds. If this fails, e.g. because I took the device with me, the ESP should start the normal sketch and write the recorded values to a SD card every 20 seconds.
If I reboot the device now, It should again attempt to connect to a WiFi. If it fails, it again writes to the SD. But if it is able to establish a connection, It is supposed to look if the SD is empty. If yes it should record the Values and send them to the Blynk server. If there is something on the SD, I want it to take all of those Values and upload them to Blynk und clean out the SD once the Values are uploaded.
Is this possible? Thanks in advance
It’s not possible to retrospectively upload data to the Blynk server with an historical time stamp.
So, you can’t record data for the last 8 hours then upload it to the Blynk server to fill-in that 8 hour gap in the recorded data.
When readings are sent to a virtual pin on the Blynk server then they are buffered for a one minute period. All readings received for that pin for the one minute period are then averaged, and the result written to the database on the Blynk server and timestamped at that point in time.
There is no way to get data into the Blynk database other than via this process.
Pete.
What values are you trying to save?
Three different Values, one is a normal int from 0 - 3000, the second one is a float value from 0.0000 to 30.0000 and the third another float from 0.00 to 99.99.
Then I would suggest that I write the values to a SD card and make a graph by plugging the SD into my PC. Do you know any software that can easily do that?