Mostly measured data in databases are defined in exact time or date. But sometimes it is necessary to interpolate between different dates because of skipped measurements. For example, in earlier times, groundwater level measurements were made "by hand" after every three to seven days, depending of site and monitoring type. But nowadays groundwater has been monitored using different loggers, which are called - divers. These little gadgets gives possibility to get daily data.
The problem is that working with daily measurements and not taking care about skipped days, statistical weights could be different from the case, when in these skipped days the measurements are interpolated.
I made a little script, where measurements and their corresponding dates are taken from mySQL database using PHP; and in the database there could be only measured data - this script interpolates the missing values.