Added datetime operators

pkg/yqlib/doc/operators/headers/datetime.md

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+# Date Time
+
+Various operators for parsing and manipulating dates. 
+
+## Date time formattings
+This uses the golangs built in time library for parsing and formatting date times.
+
+When not specified, the RFC3339 standard is assumed `2006-01-02T15:04:05Z07:00`.
+
+To use a custom format, use the `with_dtformat` operator to set the formatting context. Expressions in the second parameter then assume that date format.
+
+```bash
+yq 'with_dtformat("myformat"; .a + "3h" | tz("Australia/Melbourne"))'
+```
+
+See https://pkg.go.dev/time#pkg-constants for more examples.
+
+
+
+## Timezones
+This uses golangs built in LoadLocation function to parse timezones strings. See https://pkg.go.dev/time#LoadLocation for more details.
+
+
+## Durations
+Durations are parsed using golangs built in [ParseDuration](https://pkg.go.dev/time#ParseDuration) function.
+
+You can durations to time using the `+` operator.