NSTimeZone extension to convert GMT offset strings to NSTimeZone objects. It handles uneven (half-an-hour) offsets, like "-4.5", "+3.5", etc.. NOTE: there are no test and testing for offsets like 45 minutes and similar.

NSTimeZone+SBTExtensions.h

#import <Foundation/Foundation.h>

@interface NSTimeZone (SBTExtensions)
/*!
 @brief Convert seconds, represented as a string to the timezone. Returns nil when conversion failed.
 The method covers both, integers and floating point numbers.
 */
+ (nullable instancetype)timeZoneForSecondsFromGMTString:(nonnull NSString *)secondsString;

@end

NSTimeZone+SBTExtensions.m

#import "NSTimeZone+SBTExtensions.h"

@implementation NSTimeZone (SBTExtensions)

+ (NSTimeZone *)timeZoneForSecondsFromGMTString:(NSString *)secondsString {
    NSInteger secondsFromGMT = [secondsString floatValue] * 3600;
    NSTimeZone* timeZone = [NSTimeZone timeZoneForSecondsFromGMT:secondsFromGMT];
    return timeZone;
}

@end

NSTimeZone+SBTExtensionsTests.m

#import <XCTest/XCTest.h>
#import "NSTimeZone+SBTExtensions.h"

@interface NSTimeZone_SBTExtensionsTests : XCTestCase

@end

@implementation NSTimeZone_SBTExtensionsTests

- (void)setUp {
    [super setUp];
    // Put setup code here. This method is called before the invocation of each test method in the class.
}

- (void)tearDown {
    // Put teardown code here. This method is called after the invocation of each test method in the class.
    [super tearDown];
}

- (void)testTimeZoneForSecondsFromGMTString {
    NSString* inputString = @"+4.5";
    NSTimeZone* testTimeZone = [NSTimeZone timeZoneForSecondsFromGMTString:inputString];
    NSInteger seconds = testTimeZone.secondsFromGMT;
    XCTAssertNotNil(testTimeZone);
    XCTAssertTrue(seconds == 16200);
    
    inputString = @"+4";
    testTimeZone = [NSTimeZone timeZoneForSecondsFromGMTString:inputString];
    seconds = testTimeZone.secondsFromGMT;
    XCTAssertNotNil(testTimeZone);
    XCTAssertTrue(seconds == 14400);
    
    inputString = @"-12";
    testTimeZone = [NSTimeZone timeZoneForSecondsFromGMTString:inputString];
    seconds = testTimeZone.secondsFromGMT;
    XCTAssertNotNil(testTimeZone);
    XCTAssertTrue(seconds == -43200);
}

@end