Q Language - Temporal Data
The q language has many different ways of representing and manipulating temporal data such as times and dates.
Date
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
Times
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.
Datetimes
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.
