search by date and a time span core data - objective-c

Question

I'm working on a Core Data project, I have a table with a modification_date field,
I need to extract all the entries of the last n days that also have a modification_date between (let's say) 1PM and 9PM.
I've already sorted out part of the solution to extract just the entries in the last n days
like suggested in: Core Data- predicate with dates
NSDate *today = [NSDate date];
NSDate *end_day = [today addTimeInterval: (days*86400.0f)];
day_st = [NSString stringWithFormat: #"(timestamp >= %#) AND (timestamp <= %#) AND ", today, end_day];
My only problem now, would be adding the filter by time spans.
But, except for making a foreach to check the time of every single element in the result,
I can't come up with anything clever.

Answer 1

If you're storing date-type fields in your data model, there's no good way to do that. Those get stored as the number of seconds since a reference date-- so filtering on an NSDate works, since it's the same thing. You can't build a predicate based on date components (like hour of the day) though, because the data store doesn't know anything about those. Filtering the results after fetching them from the data store is a reasonable approach in most cases.
If this is something you'll do a lot, you might consider adding one or more new attributes-- for example, an integer field where you store the hour of the day. Then you could include those fields in your NSPredicate. Set the value for these fields any time the date field changes. And, be damn sure you know what time zone those fields are in. UTC is preferable. Local time is asking for trouble. With UTC you'd covert "1PM", "9PM", etc to their equivalents in UTC before setting values for the fields and before fetching.
Finally (and you may have realized this, but it's not obvious from your question), note that the other question you link to is building an NSPredicate while you have an NSString there.

Answer 2

I'd say you're looking to end up with a predicate like:
((timestamp >= <startTimeOnDayOne>) AND (timestamp <= <endTimeOnDayOne>)) OR
((timestamp >= <startTimeOnDayTwo>) AND (timestamp <= <endTimeOnDayTwo>)) OR
[...]
In terms of how you'd formulate the appropriate Cocoa date objects, you'd probably want to seed two NSDateComponents, one for the start time and one for the end time, and a third that simply says 'one day'. Then use an NSCalendar (just the current calendar if you're being that vague about it) and its dateFromComponents: and dateByAddingComponents:toDate:options: methods — the former to get the dates for the first day and then the latter to advance each by one day.
If you attempt just to add 24 hours at each juncture you'll get the wrong result twice a year in any territory with daylight savings.
You can build up the NSPredicates for each relevant day into an array and then use NSCompoundPredicate +orPredicateWithSubpredicates: to glue them all together into a single predicate at the end, allowing you to worry about how many is n days at runtime.