Time Zones

CTS does not use time zone names or POSIX TZ strings like TinyNTP. It sends two numbers in the Local Time Information characteristic:

Value	Unit	Example (Zurich in summer)
time zone	15 minutes, -48 .. 56	4 (UTC+1)
summer time (DST) offset	0, 30, 60 or 120 minutes (as 0, 2, 4, 8)	4 (+1 h)

Local time = UTC + time zone + DST offset. Because the device that you read tells the current offsets, it follows the user when travelling and when the clocks change.

Server

server.setTimeZone(60, 60);   // UTC+1 and one hour of summer time now

The first value is the standard time zone in minutes, the second the summer time offset that is valid now: change it when the clocks change (the server notifies the subscribed clients).

Client

cts.begin();
std::tm utc   = cts.getTm();        // UTC
std::tm local = cts.getLocalTm();   // UTC + the offsets of the device
const TinyCTSLocalTimeInfo& info = cts.getLocalTimeInfo();
info.isKnown();            // false if the device does not have the characteristic
info.utcOffsetMinutes();   // time zone + summer time in minutes

If the device has no Local Time Information, tell the client the zone yourself with cts.setLocalTimeInfo(TinyCTSLocalTimeInfo::fromMinutes(60, 0)).

UTC or local time?

The specification leaves it open if the Current Time characteristic contains UTC or the local time. TinyCTS sends UTC by default, together with the Local Time Information, so that the client can calculate the local time. Some clients (e.g. phones) expect the local time: use

• server.setTimeIsLocal(true) (before begin())
• client.setTimeIsLocal(true)
• tinycts --local

The library then converts between the two with the Local Time Information.