Use 'InvariantCulture' when converting the UNIX timestamp to string

[ahmedisam99]

In some cases (like mine) when the current culture is RTL (e.g. Arabic) and when converting a DateTime that is before the UNIX epoch it results a incorrectly formatted negative string.

// english culture
"07-15-1953" -> "-519616800000"

// arabic culture
"07-15-1953" -> "519616800000-"

Which causes issues when trying to update the value in redis