Times in GammaLib

Times in GammaLib are implemented by the GTime class that provides transparent handling of times independent of their time reference, time system and time unit. Time is stored in GTime in seconds in a GammaLib native reference system, which has zero time at January 1, 2010, 00:00:00 Terrestrial Time (TT). With respect to Coordinated Universal Time (UTC), TT time is greater than UTC time by 66.184 sec at January 1, 2010, 00:00:00. The difference is composed of leap seconds that synchronize the Earth rotation variations with a uniform clock, and of a fixed offset of 32.184 seconds between TT and International Atomic Time (TAI).

The value of a GTime instance can be set in native seconds using GTime::secs() or in native days using GTime::days(). It can furthermore also be set in Julian Days (TT) using GTime::jd(), in Modified Julian Days (TT) using GTime::mjd() and as a string in the ISO 8601 standard YYYY-MM-DDThh:mm:ss.s (UTC) using GTime::utc(). Equivalent methods exist for retrieving the time in various formats, allowing thus conversion from one format to the other. In addition, the GTime::now() method sets the time to current time of the computer, the method GTime::lmst() returns the local mean sidereal time and the method GTime::last() the local apparent sidereal time for a given geographic longitude.

As instrument times are generally given in a local reference, conversion between different time reference systems is also supported. Time references are specified by the GTimeReference class that define the Modified Julian Days (MJD) reference in TT, the time unit (seconds or days), the time system (TT or UTC) and the time reference (LOCAL). A time can be converted into a reference using the GTime::convert() method and set to a value specified in a given reference using the GTime::set() method. The native GammaLib reference can be retrieved using the GTime::reference() method.

Instances of GTime can be added, subtracted and compared using the usual mathematical operators. Subtracting two instances of time gives the differences in seconds.

Instances of GTime can be collected in the GTimes container class. This class is for the moment implemented as a minimal container class without support for reading from and writing to files.

GTime objects are also used in the definition of Good Time Intervals (GTIs), which are intervals of contiguous time during which data are valid for science analysis. GTIs are implemented by the GGti class which is a container of time intervals, formed by a start and a stop value. These values can be accessed using the GGti::tstart(int) and GGti::tstop(int) methods, both returning a GTime object. The summed length of all intervals is known as the ontime which is returned by the GGti::ontime() method in units of seconds. The elapsed time, returned by GGti::telapse() in seconds is the difference between the last stop time and the first start time. Time intervals are appended or inserted using the GGti::append() and GGti::insert() methods. These methods do not check whether intervals overlap in time, which may lead to an errornous ontime value. To remove overlaps, the GGti::merge() method can be used that will merge all overlapping intervals. GTIs can be limited to a specific interval by applying the GGti::reduce() method. GTIs can be written to or read from FITS files. The file format is the standard OGIP format used for many high energy missions. The time reference of the stored values will be defined by a GTimeReference object that can be set and retrieved using the GGti::reference() method. Time reference information is written to the FITS file in OGIP compliant header keywords.