如何将unix时间戳转换为NSData对象？

Question

I'm using Core Bluetooth to write to a peripheral. I would like to send the current unix timestamp to the sensor, and I have attempted to do that like so:

// Write timestamp to paired peripheral
NSDate*           measureTime = [NSDate date];
NSDateFormatter*  usDateFormatter = [NSDateFormatter new];
NSLocale*         enUSPOSIXLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];

[usDateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.000'Z'"];
[usDateFormatter setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];
[usDateFormatter setLocale:enUSPOSIXLocale];  // Should force 24hr time regardless of settings value

NSString *dateString = [usDateFormatter stringFromDate:measureTime];
NSDate* startTime = [usDateFormatter dateFromString:dateString];

uint32_t timestamp = [startTime timeIntervalSince1970];
NSData *timestampData = [NSData dataWithBytes:&timestamp length:sizeof(timestamp)]; // <- Troublemaker
[pairedPeripheral writeValue:timestampData forCharacteristic:currentCharacteristic type:CBCharacteristicWriteWithResponse];

Here's the issue:

My 32 bit timestamp returns the correct value, however when I convert it into NSData, the peripheral reads it as a 24 hour clock value like this: "16:42:96"

Where am I making the error?

EDIT

I have modified the code to get rid of NSDateFormatter, as someone has mentioned that it is unnecessary. I still seem to be getting the same result:

// Write timestamp to paired peripheral
NSDate*           measureTime = [NSDate date];
uint64_t timestamp = [measureTime timeIntervalSince1970];
NSData *timestampData = [NSData dataWithBytes:&timestamp length:sizeof(timestamp)]; // <- Troublemaker
[pairedPeripheral writeValue:timestampData forCharacteristic:currentCharacteristic type:CBCharacteristicWriteWithResponse]; 

Answer 1

You are confused. What you're sending to the peripheral is an integer number of seconds since 1970. That's a reasonable way to send a Unix timestamp, but it is not a time in 24 hour format, it is an integer.

You will need to change your code to use a uint64_t or uint32_t since Unix timestamps are much larger numbers than will fit in a 32 bit integer. (I suggest using uint64_t.)

(See @DonMag's comment for sample timestamp values, like 1491580283)

How you display that time once it is received by the peripheral is a separate question, and the question that you should really be asking.

Note that you may run into problems sending an int as binary data if the peripheral has different "endian-ness" than your iOS device. you might want to convert the timestamp integer to a string and send that in order to avoid the endian-ness problem.

Answer 2

There's no need to use NSDateFormatter , unless you're planning on sending a string representation to the peripheral.

From the Apple Developer docs :

NSDate objects encapsulate a single point in time, independent of any particular calendrical system or time zone. Date objects are immutable, representing an invariant time interval relative to an absolute reference date (00:00:00 UTC on 1 January 2001).

With that in mind, you can use measureTime as-is, and get the same result:

uint32_t timestamp = [measureTime timeIntervalSince1970];

Without knowing specifics about the peripheral, it's impossible to say why it's showing a 24-hour value.

If I were to hazard a guess, I would expect there to be another characteristic / value you need to modify first, in order to switch it to a different format (if that's even possible).