code: func (ctx *RequestCtx) IfModifiedSince(lastModified time.Time) bool {
  	ifModStr := ctx.Request.Header.peek(strIfModifiedSince)
  	if len(ifModStr) == 0 {
  		return true
  	}
  	ifMod, err := ParseHTTPDate(ifModStr)
  	if err != nil {
  		return true
  	}
  	lastModified = lastModified.Truncate(time.Second)
  	return ifMod.Before(lastModified)
  }
docs: // IfModifiedSince returns true if lastModified exceeds 'If-Modified-Since'
  // value from the request header.
  //
  // The function returns true also 'If-Modified-Since' request header is missing.