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Keyword Abbreviation Token (hex) Version(s) Classification
TI$ none 54 49 24 1.0+ Reserved variable
* The "token" is really just the ASCII codes for the characters "TI$"
Returns Type Value(s) Note(s)
timeHMS String
6 digits:
000000 ~ 235959
HHMMSS format
(hours, minutes, seconds) 
Get or Set the system time.
This is the string version of the BASIC time variable.  The timeHMS is always a 6-character string.  Unlike all other reserved variables, this one may be assigned a value.  When not assigning a value (get the time), the timeHMS will always consist of numeric digits that range from 000000 to 235959 where the first two digits are hours (0~23), the middle two are the minutes (0~59), and the last two are the seconds (0~59).
In BASIC versions before 3.5, the timeHMS (when assigning) may include non-digits.  In such a case, the value of that "digit" would be calculated as ASC(digit)-48.  This gives the expected 0 ~ 9 for actual numeric digits, 10 for @, and 11 ~ 36 for unshifted letters A ~ Z (these are examples, any character is fine).  In versions 3.5 and beyond, attempts to use non-numeric digits generate ILLEGAL QUANTITY ERROR.
When assigning, if the seconds or minutes part of timeHMS exceed 59 then that part will be interpretted as zero (in version 1.0) or as value-60 (in versions 2+), and the next part (minutes or hours, respectively) will be increased by one.  If the hours part exceeds 23, then the system time is set to zero (000000), after a Jiffy (see examples).  In othe words, wrapping of values is broken in 1.0, and in other versions it works for seconds and minutes but not hours.
If you attempt to assign a string of not exactly 6 characters, ILLEGAL QUANTITY ERROR is generated.  Attempts to assign a numeric expression (a non-string) will generate TYPE MISMATCH ERROR.
This string version of the system time is only accurate to the nearest second, although the system time is mainted by a "Jiffy Clock" which has a maximum resolution of 1/60 second.  You can read this higher precision value with the numeric equivalant, TI, but you can not set that reserved variable.
Like all BASIC variables, only the first two characters of the name are significant.  So you may also use (and early documentation shows) TIME$ if you prefer (note the M and E are superfluous).
On most (all?) of the CBM 8-bits, the system time is mainted by software in the KERNAL, even though some machines have hardware which is more reliable.  In particular, delays in communicating with external devices (and a few internal operations) will cause the system time to "fall behind" reality (i.e., run slow).  Generally the more I/O operations that take place, the less reliable the system time.
The C128 has FAST and SLOW commands to change the CPU speed, and thus the speed of BASIC execution, but the Jiffy Clock works the same at either speed.
Unfortunately, none of the CBM BASICs have a DATE$ variable or function.  So you would need to periodically check the TI$ for "rollover" to detect a change in the date.
TI$ = "000000" : PRINT TI$

TI$ = "000000" : FOR I = 1 TO 1000: NEXT : PRINT TI$
000001         value may vary a bit on different machines

TI$ = "000059" : FOR I = 1 TO 1000: NEXT : PRINT TI$
000100         value may vary a bit on different machines

TI$ = "000099" : PRINT TI$
000100         in V1.0
 ~ or ~
000139         in V2+

TI$ = "00000A" : PRINT TI$
000017         in V1.0 ~ 2.x
 ~ or ~

TI$ = "239999" : PRINT TI$ : PRINT TI$

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