Blacktime

Blacktime is a Whitespace derivative designed and originally implemented in 2014 by Three If By Whiskey. It was inspired by the realization that Whitespace's 24 distinct operations would map very well into a system of encoding instructions as times.

Syntax
_ _  _  _  |_||_||_||_| |_||_||_||_|

Blacktime programs are composed entirely of spaces (' '), underscores ('_'), vertical pipes ('|'), and line breaks. Instructions are written as contiguous groups of four seven-segment displays, representing either a complete 24-hour time or a "time delta", explained below. There are no syntax errors; if the character in a given position is not the underscore or pipe expected for that position, it is simply ignored.

Evaluation
The initial instruction in a Blacktime program (the "seed") must be a complete, zero-padded, 24-hour time. Each subsequent instruction is a "time delta" whose segments are XOR'd with the previous time's to produce the current time. It is a runtime error, and program execution terminates, if at any point an invalid time is generated by this process.

The previous hour is subtracted from the current, and this value (modulo 24, with the result taking the sign of the divisor) indicates which operation to perform. This same calculation is applied to the minutes (modulo 60) to produce the argument supplied to the operation, if applicable.

Operations
The first eight operations are the ones which take an argument, denoted as n in their descriptions.

Hello, world!
_ _     _                    _              _                          _                 _  _  _  _         _|| ||_||_|                    |         | ||_             |_  _|      |                  _ |_||  |_|       |_ |_|  ||_|                   _             |_       | |     ||        |_ |              |_| _ |_ |_         _  _                    _  _           _              _                 _                       _  _        |   _                | | _  _       |_ |_             |   _           _ |                  _|   |            |_ |_|               | ||_||_|        ||_             |_ |_|          _ |_ |              |     |_  _         _  _              _     _                    _        _  _                    _     _                       |    |            |_|   |  | |            |_ |        |              |         _ |  |                    |   |_  _              _ | ||_ | |               |_|      |_||_          |        |_|   |_ | |                    _                       _                 _              _  _                    _  _  _        _            _          |_ |        |                 |            _||  |    |            | | _ |    |      |  |_        |_|| |        || |      |_ |              |_          |   _ |_ |                 |_ |_  _       |_                        _  _                                _                       _              |_       |  |           |_                    _          |_  _       |    |                   |_  _          |                    |_|           |         |_ |

Cat
_       _     _  `push 0` operations              _ _| |  | _||_ |   are good places to   _                   |_ |_ |_  |  | _|  ||_  store commentary. |           _         |              _        _        _                                   _              |_ |_|      |  | |   |     |_           _|           |     |   |_          ||_       |_ |      _    |           |           |      _    |_

Computational class
Because it is possible to interpret brainfuck in Whitespace, Blacktime is trivially Turing-complete.