Timestamps pt 4 – Adaptive Time Base

When dates and times are manually entered we may need only some of the following: year, month, day, time zone, hour, minute, second. Although the timestamp may have millisecond resolution, high resolution data may be inconsequential. For example a list of birthdays requires only year, month and day. Time zone, hour, minute, second and milliseconds are no factor and are implicitly set to zero. When exchanging data with a server or another application it would be desirable to send only the data we need and no more. This can be done with an adaptive time base.

Color My Data proposes an Eight-Bit Compressed Binary Format (CBF-8) coder / decoder which generates a policy indicator (DATETIME) followed by zero or more base-64 digits as follows:

  • digit 1 – years range -32 to +31 step 64 years bias 2000
  • digit 2 – years range 0 to 63 step 1 year
  • digit 3 – months range 1 to 12 step 1 month
  • digit 4 – days range 1 to 31 step 1 day
  • digit 5 – time zone range -24 to +24 step 30 minutes
  • digit 6 – MSB time zone range 0 to 1 step 15 minutes
  • digit 6 – five bits hour range 0 to 23 step 1 hour
  • digit 7 – minutes range 0 to 59 step 1 minute
  • digit 8 – seconds range 0 to 59 step 1 second
  • digit 9 – subseconds range 0 to 62 step 16 milliseconds
  • digit 10 – subseconds range 0 to 63 step 250 microseconds
  • digits 11 to n – subseconds range 0 to 63 step 250 x 6410-n microseconds.

The DATETIME policy causes CBF-8 to use MSB alignment which ignores trailing zeros. As trailing zeros increase, both the number of encoded digits and the effective clock frequency decrease. Returning to the birthday example, a four-byte (digit) stream may generate any birthday between Jan 1 28 BC and Dec 31 4047 AD with a resolution of one day. A seven-byte (digit) increases the clock rate from one cycle per day to one cycle per minute by adding time zone, hours and minutes (and nothing more). On the other extreme increasing the clock rate to 2GHz (500 picosecond period) would typically take 15 digits.

The input arguments for encoding timestamps are the minute of epoch 2000-01-01T00:00Z mm2K() and sec() from the previous post and the time-zone, a value between -48 and +48 that scales to -12:00 to +12:00 in 15 minute increments.

CBF8#append(OutputStream sink, int mm2K, double sec, byte timeZone);

The CBF8Decoder supports verification of the DATETIME policy, access to the array of base-64 digits and the number of encoded digits. This greatly simplifies the conversion to human readable formats. The timestamp class uses these data to decode mm2K and sec.

int mm2K(CBF8Decoder);
double sec(CBF8Decoder);

Advantages:

  • encoded data are portable across all implementations
  • timestamp splits into year, month, day, hour, minute, second and subsecond
  • split data is easy to format for human readability
  • encoded data require fewer bytes than formatted text
  • only significant digits are encoded
  • number of digits adapts to clock rate

Disadvantages:

  • Computationally intensive relative to addition / subtraction due to numerous integer remainder and integer divide operations.
  • Base-64 digits above F (15) not easy to relate to decimal digits
  • .

Suggested Use Cases:

Peer to peer and client-server data exchanges. Splitting data for formatted data input / output.