Convert mysql irregular time series data to regular sequence - php

Question

I have a table of temperature data, updated every 5-15 mins by multiple sensors.
The data is essentially this: unique id, device(sensor id), timestamp, value(float)
The sensors does not have an accurate clock, so the readings are doomed to skew over time, so I'm unable to use things like group by hour in mysql to get a reading of the last 24h of temperature data.
My solution as a php programmer would be to make a pre-processor that reads all the un-processed readings and "join them" in a table.
There must be others than me who has this need to "downscale" x-minute/hour reads down to one per hour, to use in lets say graphing.
My problem is how do I calculate the rounded hour value from one or several readings.
For example, I have 12 readings over 2,5 hours, and I need an explicit value for each whole hour for all these readings.
Data:
Date Device Value
2016-06-27 12:15:15, TA, 23.5
2016-06-27 12:30:19, TA, 23.1
2016-06-27 12:45:35, TA, 22.9
2016-06-27 13:00:55, TA, 22.5
2016-06-27 13:05:15, TA, 22.8
2016-06-27 13:35:35, TA, 23.2
I'm not that much into statistical math, so "standard deviation" and the likes are citys in Russia for me.
Also, the devices go to sleep sometimes, and does not always transmit a temperature.
Feel free to ask me to add info to the question, as I'm not sure what you guys need to answer this.
The most important parts is this:
1. I'm using MySQL, and that's not going to change.
2. I'm hoping for a solution (or tips) in php, though tips in many other languages also would help my understanding. I'm primarily a PHP programmer though, so answers in that language would be most appreciated.
Edit: I would like to specify a few points.
Because the time data recorded from the sensors may be inaccurate, I'm relying on the SQL insert time. That way the time is controlled by one device only, the controller that's inserting the data.
For example, if I select 30 timestamp/value pairs in a 24h period, I would like to "combine" these to 24 timestamp/value pairs, using an average to combine the overflowing data.
I'm not that good to explain, but I hope this makes it clearer.
Also, would love either a clean SQL way of doing it, but also a PHP way of looping through 30 rows to produce 24 whole hour rows of data.
My goal is to have one row for every hour, with an accurate timestamp and temperature value. Mainly because most graphing libraries expect that kind of input. Especially when I have more than one series in a graph.
At some point, I may find it useful to show a graph for let's say the last six hours, with a 15 minute accuracy.
The clue is that I don't want to change the raw data, just find a way to extract/compute linear results from it.

Answer 1

How I would try to handle this is;
Take day start value; 01/01/2016 00:00:00 and do a 'between' 'sql' in MySQL, progressing every hour. So the first 'sql' would be like;
'select avg(temp_value) from table where date between 01/01/2016 00:00:00 and 01/01/2016 00:59:99' and progress on by the hour.
The sql isn't correct, and the entire 24hr period can be written out programmatically, but I think this will start you on your way.