Marc Niegowski Systems Programmer Consultant - 370, 390, z Assembler STCK UTC or GMT to LOCAL Time Conversion

STCK format date and time values are often either synchronized to UTC (Universal Coordinated Time) or GMT (Greenwich Mean Time). UTC has a fixed length second that is periodically corrected by inserting leap seconds, while GMT has a variable length second. For more information regarding the differences between UTC and GMT refer to the z/Architecture Principles of Operation.

The z/OS operating system maintains the Time Zone and Leap Second offsets in the OS/VS2 Common CVT Extension control block. In order to convert a UTC STCK value or a GMT STCK value to a local time STCK value, one must apply these offset values to STCK value.

Here's the first code snippet of two illustrating a technique that will convert either UTC or GMT to Local time:

. . . L R1,CVTPTR Load A(CVT). USING CVT,R1 Establish CVT addressability. * L R1,CVTEXT2 Load A(CVT Extension). DROP R1 Drop CVT. * USING CVTXTNT2,R1 Establish Ext. addressability. * STCK STCKTOD Store current UTC/GMT clock value. LM R2,R3,STCKTOD Load clock Hi and Lo words. LM R4,R5,CVTLDTO Load offset Hi and Lo words. ALR R3,R5 Add Lo word time offset. ALCR R2,R4 Add w/Carry Hi word offset. * * CVTLSO High and Low words should both be zero if the clock * is synchronized to GMT so the following instructions will * only have an effect if the clock is synchronized to UTC. * LM R4,R5,CVTLSO Load leap sec Hi and Lo words. DROP R1 Drop CVT Extension. * SLR R3,R5 Sub Lo word leap seconds. SLBR R2,R4 Sub w/Borrow Hi word leap secs. STM R2,R3,STCKTOD Store local time clock value. . . . STCKTOD DC FL8'0' STCK work field. . . . CVT DSECT=YES,LIST=YES Generate CVT DSECT. . . .

In the above example, we convert either a UTC or GMT 8 byte clock value to a 8 byte local time clock value using 32-Bit registers.

The following code snippet saves a couple of instructions over the previous example by using 64-Bit z/Architecture registers:

. . . L R1,CVTPTR Load A(CVT). USING CVT,R1 Establish CVT addressability. * L R1,CVTEXT2 Load A(CVT Extension). DROP R1 Drop CVT. * USING CVTXTNT2,R1 Establish Ext. addressability. * STCK STCKTOD Store current UTC/GMT clock value. LG R2,STCKTOD Load 64-Bit clock value. LG R3,CVTLDTO Load 64-Bit date/time offset value. ALGR R2,R3 Add 64-Bit date/time offset. * * CVTLSO High and Low words should both be zero if the clock * is synchronized to GMT so the following instructions will * only have an effect if the clock is synchronized to UTC. * LG R3,CVTLSO Load 64-Bit leap second offset value. DROP R1 Drop CVT Extension. * SLGR R2,R3 Subtract 64-Bit leap second offset. STG R2,STCKTOD Store 64-Bit local time clock value. . . . STCKTOD DC FL8'0' STCK work field. . . . CVT DSECT=YES,LIST=YES Generate CVT DSECT. . . .

If for some reason, no Time Zone offset is used (i.e., the LPAR time is synchronized to local time), both CVTLDTO and CVTLSO will contain zeroes. If the LPAR time is synchronized to GMT, only CVTLDTO will contain an offset and CVTLSO will contain zeroes. If the LPAR time is synchronized to UTC, CVTLDTO will contain the time and date offset and CVTLSO will contain the leap second offset. Thus, both the latter code snippets work regardless how the LPAR time is synchronized.