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Q Language - Temporal Data
The q language has many different ways of representing and manipulating temporal data such as times and dates.
A date in kdb+ is internally stored as the integer number of days since our reference date is 01Jan2000. A date after this date is internally stored as a positive number and a date before that is referenced as a negative number.
By default, a date is written in the format “YYYY.MM.DD”
q)x:2015.01.22 / This is how we write 22nd Jan 2015 q)`int$x / Number of days since 2000.01.01 5500i q)`year$x / Extracting year from the date 2015i q)x.year / Another way of extracting year 2015i q)`mm$x / Extracting month from the date 1i q)x.mm / Another way of extracting month 1i q)`dd$x / Extracting day from the date 22i q)x.dd / Another way of extracting day 22i
Arithmetic and logical operations can be performed directly on dates.
q)x+1 / Add one day 2015.01.23 q)x-7 / Subtract 7 days 2015.01.15
The 1st of January 2000 fell on a Saturday. Therefore any Saturday throughout the history or in the future when divided by 7, would yield a remainder of 0, Sunday gives 1, Monday yield 2.
Day mod 7 Saturday 0 Sunday 1 Monday 2 Tuesday 3 Wednesday 4 Thursday 5 Friday 6
A time is internally stored as the integer number of milliseconds since the stroke of midnight. A time is written in the format HH:MM:SS.MSS
q)tt1: 03:30:00.000 / tt1 store the time 03:30 AM q)tt1 03:30:00.000 q)`int$tt1 / Number of milliseconds in 3.5 hours 12600000i q)`hh$tt1 / Extract the hour component from time 3i q)tt1.hh 3i q)`mm$tt1 / Extract the minute component from time 30i q)tt1.mm 30i q)`ss$tt1 / Extract the second component from time 0i q)tt1.ss 0i
As in case of dates, arithmetic can be performed directly on times.
A datetime is the combination of a date and a time, separated by ‘T’ as in the ISO standard format. A datetime value stores the fractional day count from midnight Jan 1, 2000.
q)dt:2012.12.20T04:54:59:000 / 04:54.59 AM on 20thDec2012 q)type dt -15h q)dt 2012.12.20T04:54:59.000 9 q)`float$dt 4737.205
The underlying fractional day count can be obtained by casting to float.