Siemens SIMATIC S7 System Manual page 186

Program instructions
7.2 Clock
Format of 8 byte time buffer, beginning at byte address T
You must assign all date and time values in BCD format (for example, 16#12 for the year
2012). The BCD value range of 00 to 99 can assign years, in 2000 to 2099 range.
T byte
0
1
2
3
4
5
6
7
Extended power outage effect on the CPU clock
See the S7-200 SMART system manual appendix A CPU specifications, for how long the
real-time clock can maintain the correct time during power outages.
A CPU initializes with the time values shown in the following table, after an extended power
outage.
Date
01-Jan-2000
Note
Compact serial (CRs) CPU models do not have a RTC (Real-time Clock)
You can use the READ_RTC and SET_RTC instructions to set the year, date, and time
values in compact serial (CRs) CPU models, but the values will be lost on the next CPU
power off-on cycle. On power-up, the date and time will initialize to January 1, 2000.
186
Description Data value
Year 00 to 99 (BCD value) Year 20xx: where xx is the two digit BCD value in
T-byte 0
Month 01 to 12 (BCD value)
Day 01 to 31 (BCD value)
Hour 00 to 23 (BCD value)
Minute 00 to 59 (BCD value)
Second 00 to 59 (BCD value)
reserved Always set to 00
Day of week Value ignored when written with the SET_RTC / TODW instruction.
Value reports correct day of week when read with the READ_RTC /
TODR instruction based upon current Year/Month/Day values.
1 to 7, 1 = Sunday, 7 = Saturday (BCD value)
Time
00:00:00
Day of week
Saturday
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART