Seems to be a bug in subtracting days

[TheBeginner]

subtraction doesn't work (says second parameter invalid) but adding does

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test := A_Now
test -= 1 , Days ; Doesn't work

; While this Does
test += 1 , Days ; works

bug?

[flyingDman]

No bug. Read the docs*. (EnvSub and EnvAdd)

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test := A_Now
test += -1 , Days ; works
msgbox % test

* can the docs be clearer? yes!

[TheBeginner]

this is an actual example from the docs

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Var -= Value , TimeUnits

if you use that example it will fail

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Var1 -= 1, days

plus since this is how it works in the += why would it work differently in the subtraction?
is there anything in the docs mentioning a exception for the subtraction method, I've read them and I didn't find any, if you see something listed in the docs mentioning why this doesn't work do point me to it.

also note that example number two, is

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MyCount -= 2

which is exactly the same, minus a TimeUnits parameter.

so either it's a bug or the documentation needs improvement

[boiler]

this is an actual example from the docs

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Var -= Value , TimeUnits

if you use that example it will fail

It doesn't fail when used as intended which is subtracting one date-time stamp from another.

is there anything in the docs mentioning a exception for the subtraction method, I've read them and I didn't find any, if you see something listed in the docs mentioning why this doesn't work do point me to it.

The EnvSub documentation states:

If present, this parameter directs the command to subtract Value from Var as though both of them are date-time stamps in the YYYYMMDDHH24MISS format.
...
To add or subtract a certain number of seconds, minutes, hours, or days from a timestamp, use EnvAdd (subtraction is achieved by adding a negative number).

What you are trying to do is different than two date-time stamps, and the documentation makes a distinction between finding the difference between two date-time stamps and subtracting a number of time units from a date-time stamp. Yes, it can be more clear, but it is technically correct.

[TheBeginner]

I think what threw me off is that I thought The timeunit in

Var += Value , TimeUnits

Was to tell AutoHotkey what's the time unit in the Value so it can Add or Subtract from the date-time stamp, but it seems that the timeunit is there to indicate in which units the Var will be stored

Is my understanding correct?

[flyingDman]

Think of it this way: if you want to add or subtract a number of time units (days, minutes, etc) from a date, use EnvAdd or Var += Value, TimeUnits. The result var will de a date.
If you want to subtract a date from another date (to calculated the date difference or the difference between 2 timestamps), use c]EnvSub[/c] or Var -= Value, TimeUnits. The result Var will be a number of time units.