Is there a way to convert a full date to milliseconds?

Milliseconds since when? You can get the value as an Unix timestamp using now()->timestamp

Use microtime(true) instead of now() to get the timestamp in milliseconds.

For example, I got this

date: 2020-07-06 20:17:28.900295

I want to convert the whole date to milliseconds or something that can be compared to another date like that.

Use microtime(true) instead of now() to get the timestamp in milliseconds.

No., this returns in seconds with milliseconds (as a decimal value). With both solutions given above, the OP would need times 1000 the value returned because they both return seconds

$millisecondsSinceEpoch = microtime(true) * 1000

Edit @chron Carbon has a getPreciseTimestamp method which takes a precision argument (3 give millisecond precision):

$numMilisecondsSinceEpoch = Carbon::parse('2020-07-06 20:17:28.900295')->getPreciseTimestamp(3);

If you need to compare date/times, Carbon has this built-in:

now()->gte(Carbon::parse('2020-01-01 01:02:03'));
// true

now()->lt(Carbon::parse('2020-01-01 01:02:03'));
// false