# MTK & Standard Library & independent variable, etc
**URL:** https://discourse.julialang.org/t/mtk-standard-library-independent-variable-etc/104885
**Category:** Modelling & Simulations
**Created:** [October 12, 2023, 7:01am UTC](https://discourse.julialang.org/t/mtk-standard-library-independent-variable-etc/104885 "2023-10-12T07:01:30Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@BLI](https://discourse.julialang.org/u/BLI)
#### Post date: [October 12, 2023, 7:01am UTC](https://discourse.julialang.org/t/mtk-standard-library-independent-variable-etc/104885/1 "2023-10-12T07:01:30Z")
Thanks for all help with MTK so far – I’m learning quite a bit just by posing the questions…
In Modelica, the independent variable has to be denoted `time`, and time differentiation is `der()`.
In MTK, the choice of independent variable and differentiation operator is more flexible, partly due to MTK having a wider scope (e.g., also handling PDEs).
**Questions** [somewhat overlapping]:
- Is there a standard choice of time variable and time differentiation in MTK Standard Library? E.g., `@variables t` and `D = Differential(t)`?
- If so, should these be defined in the REPL/global namespace? What happens if I, e.g., use `Dt = Differential(t)` in my own “re-usable” model units? Will that cause a problem? If `D = Differential(t)` is _assumed_ in MTK SL blocks, will that leave `D` undefined?
- … if I define `Dt = Differential(t)` in my own units, that causes no problems if the differential `D = Differential(t)` is explicitly defined in the MTK SL unit/block, right?
- It would be more problematic to choose different names for the independent variable for time in different units such as my own units and those from MTK SL, right? Suppose MTK SL uses `t` for time and I for some strange reason go for \tau in my own unit, I assume that will cause a problem?
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### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [October 12, 2023, 8:06am UTC](https://discourse.julialang.org/t/mtk-standard-library-independent-variable-etc/104885/2 "2023-10-12T08:06:27Z")
> [@BLI](#):
>
> Is there a standard choice of time variable and time differentiation in MTK Standard Library? E.g., `@variables t` and `D = Differential(t)`?
Not yet, but we’ll be changing this soon so there is a standard.