you can do this with a query to identify records which do not have one that follows in 5 minutes by joining the table to itself on location.
However as with this post with much the same question
https://www.accessforums.net/showthr...acr%2C+missing
you need to be clear about the data you have. Your tablename would appear to imply a single location, but that infringes normalisation best practice. but based on what you have provided - i.e. there is only one location/measuring point in the table the solution might be as below. Also be aware the code provided below only looks for a single instances of a missing time - if there are two or more missing times next to each other, only the first one is reported - and of course the very last record will be reported since there is no later record. So as Bob says - you need to be clear about what it is you want reporting.
Code:
SELECT dateadd("n",5,S.measurementtime) AS Missing
FROM Weatherstation AS S LEFT JOIN Weatherstation AS E ON dateadd("n",5,S.measurementtime)= E.measurementtime
WHERE E.measurementtime is null
Note this uses a non standard join so cannot be created in the query builder, you need to use sql. But there is a small cheat. In the query builder, just join on the measuremetime field and then go to sql builder and replace the join.
You say you have 1.5m records - so make sure your measurementtime field is indexed otherwise it will be painfully slow
Time can also be quite difficult to join on because it is stored as a decimal number - the number of seconds divided by 86400, the number of seconds in a day - so 5 mins=300 seconds/86400 is a pretty small number and you might just get some rounding differences - you'll need to try it and see.