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 canonical units:: Representative units of the physical quantity.
 Unless it is dimensionless, a variable with a **`standard_name`** attribute must have units which are physically equivalent (not necessarily identical) to the canonical units, possibly modified by an operation specified by the standard name modifier (see below and <<standard-name-modifiers>>) or by the **`cell_methods`** attribute (see <<cell-methods>> and <<appendix-cell-methods>>) or both.
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 Units of time coordinates (<<time-coordinate>>), whose **`units`** attribute includes the word **`since`**, are _not_ physically equivalent to time units that do not include **`since`** in the **`units`**.
 To mark this distinction, the canonical unit given for quantities used for time coordinates is **`s since 1958-1-1`**.
 The reference datetime in the canonical unit (the beginning of the day i.e. midnight on 1st January 1958 at 0 `degrees_east`) is not restrictive; the time coordinate variable's own **`units`** may contain any reference datetime (after **`since`**) that is valid in its calendar.